Gemara

24b (Rav Nachman): If a woman miscarried a Shefir (the outer skin of a fetus before bones form), she is a Safek Yoledes.

25b (Beraisa - Aba Sha'ul bar Nash): We take a chip of wood with a smooth end and move it where the genitals should be. If it gets caught on something, it is a boy. If not, it is a girl.

Chulin 17b (Chachamim of Sura): Since the knife cuts flesh, it must be checked with flesh.

(Rav Papa): The knife must be checked with the flesh (finger) and fingernail, and on three sides.

Ravina: I heard you (Rav Ashi) quote Rava to say that the knife must be checked with the flesh and fingernail, and on three sides.

Version #1 - Rav Ashi: I said that it must be checked with the flesh and fingernail. I did not say it must be checked on three sides.

Version #2 - Rav Ashi: I said that it must be checked with the flesh and fingernail, and on three sides, but I did not say this in Rava's name.

Rav Acha brei d'Rava checked a knife with his flesh and fingernail and on three sides. Rav Ashi approved. Also Rav Kahana agrees.

(Rav Yemar): It must be checked with the flesh and fingernail, but it need not be checked on three sides;

Didn't R. Zeira teach that Shechitah with a glowing hot knife it Kosher, for the blade cuts before burning through the Simanim!

Question: It burns the Simanim on the sides (of the blade, as it cuts)!

Answer: When the Simanim are cut, the sides of the cut immediately separate from each other (so the blade does not touch them).

Also regarding a cold knife, even if it has a nick on the side, it will not touch the Simanim!

Rishonim

Rambam (Hilchos Shechitah 1:23): One who slaughters must check the knife on the blade and both sides. He is Molich and Mevi (slides away and pulls back) the knife along the skin of his finger, and he is Molich and Mevi it along his fingernail from three directions, i.e. the blade and both sides, to ensure that there is no nick at all.

Rosh (Chulin 1:8): The Rif did not bring R. Zeira's law. Also the Rambam (1:22) disqualifies Shechitah with a glowing hot knife. This is because Rav Yemar requires the flesh and fingernail, but not three sides. He learns from R. Zeira, who holds that the sides of the Simanim separate from each other when the Siman is cut. We hold like Rav Ashi, who requires three sides. If so, we do not hold that the sides of the Simanim separate. Therefore, we must be concerned for the sides of the knife. It can make a Terefah. The Gemara said 'didn't R. Zeira say...' R. Yonah derives that it was known that the Halachah follows him. Perhaps we hold that the sides separate from each other, so they are not burned. However, a protrusion sticks out and extends to the side of the Siman, and tears it. The Gemara did not say so, for this distinction was obvious. Also, there he knows that the knife was glowing hot, so he is careful not to tilt it to the side. If one does not check the knife for a protrusion on the side, sometimes there is a protrusion and he does not know about it, so he is not careful not to tilt. Also the Ra'avad says that R. Zeira permits only b'Di'eved, but not l'Chatchilah. Therefore, one must check on three sides. We do not rely on him to be careful not to tilt. Similarly, we forbid a Tamei to slaughter Kodshim, lest he touch the meat. All the more so here it is forbidden (l'Chatchilah), for he does not know about the protrusion, and he is not careful not to tilt. Even b'Di'eved, if the knife was not checked on three sides, it is Pasul b'Di'eved, lest he tilted.

Rosh (24): The Halachah is, one must check with the flesh, fingernail, and on three sides. One must check with flesh because the Veshet (foodpipe) is soft. One must check with a fingernail because the Kaneh (windpipe) is hard. One must check on three sides, lest there is a protrusion on the side of the knife, and one does not feel it when he checks the blade. The protrusion tears the Simanim on the side. We do not say that the Beis ha'Shechitah (Siman) expands regarding a protrusion on the side. One must check separately with the flesh and fingernail, for one does not feel it well if he checks with them at the same time. Similarly, one may not check both sides at the same time. One must check a total of 12 times (Molich and Mevi, with flesh and fingernail, on each of three directions). A Siman is "v'Shachatetem b'Zeh." (The Gematriya of b'Zeh is 12.)

Poskim

Shulchan Aruch (YD 18:4): If when one checks a knife through Holachah he does not feel a nick, but during Hava'ah he feels a nick, this is called Mesuchseches. If he slaughtered only through Holachah, without Hava'ah, it is Kosher

Taz (3, Shach 6, Gra 8 and Drishah): If he holds the knife in one hand with the blade up, and he is Molich the finger along the blade, this Sa'if does not make sense! (Such a nick would tear through Holachah,but not through Hava'ah! - PF) Rather, he is Molich and Mevi the blade along the finger. Sa'if 9 explicitly says that this is how they checked.

Shulchan Aruch (9): The knife must be checked with the flesh and fingernail, and on three sides. I.e. he is Molich and Mevi the knife along the skin of his finger, and afterwards he is Molich and Mevi it along his fingernail from three directions, i.e. the blade and both sides, to ensure that there is no nick at all.

Beis Yosef (DH uv'Inyan): The Tur connotes that the Gemara concluded that we require the flesh, fingernail, and on three sides. The Gemara did not conclude! Further, some Poskim rule like Rav Yemar, who says that b'Di'eved one need not check on three side, due to R. Zeira's law. The Tur saw several late Amora'im who require flesh, fingernail and three sides, therefore he wrote that this is the conclusion. All the famous Poskim rule like this. One must check with flesh because the Veshet (foodpipe) is soft. One must check with a fingernail because the Kaneh (windpipe) is hard.

Taz (6): One must check with flesh due to the Veshet, and with a fingernail due to the Kaneh. Anything that these (the flesh and fingernail) do not feel, the Veshet and Kaneh will not feel a protrusion. Tosfos says so.

Note: Tosfos (17b DH a'Bisra) said only the first sentence of the Taz.

Gan ha'Melech (57, cited in Pischei Teshuvah 4): If one slaughtered one Siman of a bird, e.g. the Veshet, and he checked the knife only with flesh, it is Kosher. Even if there was a nick that a fingernail would feel, or vice-versa (he slaughtered the Kaneh, and he checked only with a fingernail), it is fine. Rabanan explained that one must check with flesh due to the Veshet, and with a fingernail due to the Kaneh.

Pischei Teshuvah (4): Me'ir Nesivim (36) strongly disagrees, and calls this astounding.

Tosefes Merubah (Otzar Meforshim 7, in Shulchan Aruch ha'Shalem): The finger can feel small nicks that the nail cannot feel. The nail can feel a nick at the end of the knife, where the finger cannot enter.

Note: Clearly, also he argues with Gan ha'Melech. Perhaps he explains that the Gemara gave one reason to require flesh and fingernail, but there are also reasons.

Aruch ha'Shulchan (12): Our custom is to pass the nail over the knife. Also the Rosh says so. One of the great Acharonim (Ma'adanei Yom Tov on Rosh 1:24, Os 9 and 30) learns from the Gemara that one must pass the knife over the nail. It says that one checks a'Bisra va'Tufra (on the flesh and fingernail), and not b'Bisra uvi'Tufra (with the flesh and fingernail). Also the Rambam and Shulchan Aruch say on the finger and fingernail. Reasoning supports this. The knife passes over the neck during Shechitah, so checking should resemble this. He was astounded that the Beis Yosef did not mention the difference between the Tur and Rambam. I say that it makes no difference. One may do however he feels it better. Surely, in the days of the Gemara they passed the knife over the nail. This is clear from the Sugya of a nick that tears in only one direction (17b). The Taz (3) says so. However, we feel it better when the nail passes over the knife than when the nail is stationary, like it says in Nidah. One moves a chip of wood (over the Shefir. We do not move the Shefir over the chip.) One who feels it better when the knife passes over the nail should do so (Mishbetzos Zahav 3). This is why the Shulchan Aruch did not address the difference between the Tur and Rambam. It is clear from the words of the Rosh and Tur that in Ashkenaz, the custom was to pass the finger over the knife. Perhaps it is because there it is cold in winter, so the hand gets hard and does not feel, so one must warm the hand. The Chachamim of the Gemara were in Eretz Yisrael, and the Rambam was in a warm land, so there was no need for this. It sufficed to pass the knife over the finger. If so, we (Ashkenazim) should not deviate from our custom.

Shulchan Aruch (ibid.): He must check slowly with intent, lest he divert his mind to other matters. He must use a different fingernail after part of the checking, lest the blade make a groove in his nail, and perhaps there will be a nick on the side and he will not feel it because it passes in the groove.

Source: Kol Bo (107:1 80a, citing the Ra'avad), brought in Beis Yosef (DH v'Chasav ka'Kol).

Shach (14): Mishmeres ha'Bayis (7a, citing the Ra'avad) says that one must wash the knife before checking it, for sometimes blood is congealed in the nick. The Ra'avad testified that a case occurred in which he checked and did not find a nick, and afterwards when he shook the knife in water, he found a nick.

Be'er Heitev (15): If one checked a knife only on the blade, even if he slaughtered many animals and the knife was lost, Damesek Eliezer is Machshir. Devar Shmuel (200) says that one may check with any finger he wants. Lechem ha'Panim (14) says to use only the middle finger. The Pri Chodosh (14) says to wet the nail with a little saliva, for also Simanim in a live animal are wet, even though one feels it better without saliva.

Pischei Teshuvah (5): Michtav Eliyahu says not to check the knife when the hand is cold or very hot, or if the hand is wet from a liquid or sweat, even if it is not wet enough to wet something else, for in all such cases he does not feel. Also, he should clean the knife so there is not any moisture. Also, one should not check outside when the wind is blowing, for the wind harms (ability to check).

Rema: He may not check both sides at once. Rather, he checks each by itself.