Gemara

(Mishnah): Witnesses can testify only if they saw face, including the nose. This is even if there are Simanim on his body and clothing.

Inference: This shows that mid'Oraisa we may not rely on Simanim.

Contradiction (Mishnah): If (a Shali'ach to give a Get lost the Get, and) it was found tied to his wallet or ring, or if it was found among his clothes, even after a long time, the Get may be used.

Answer (Abaye): Tana'im argue about whether or not mid'Oraisa we may rely on Simanim:

(Beraisa): We may not identify a dead man based on a wart to permit his wife to marry;

R. Eliezer ben Mahavoy permits this.

Suggestion: Chachamim hold that we may rely on Simanim mid'Oraisa. R. Eliezer says that it is only mid'Rabanan.

Version #1 - Rejection (Rava): All hold that Simanim are mid'Oraisa;

Version #1a: They argue about whether we are concerned lest someone born at the same time has an identical wart.

Version #1b: They argue about whether or not warts are prone to change after death.

Version #2 - Rejection (Rava): All hold that (regular) Simanim are mid'Rabanan. They argue about whether or not a wart is an exceptional Siman (which works mid'Oraisa).

Question: In Version #1, Rava says that Simanim are mid'Oraisa. But the Mishnah says (that witnesses can testify only if they saw the face) even if there are Simanim on his body and clothes!

Answer - part 1: The Simanim on his body are (vague and unreliable, e.g.) he is tall or short;

Answer #1 (regarding clothing): We cannot rely on Simanim on his clothing, for we are concerned lest someone borrowed his clothing.

Question: If we are concerned that someone borrowed his clothing, why do we return a donkey based on a sign on the Ukaf (wood placed above the saddle)? Question: If a Get was found tied to a man's wallet or ring, why may it be used? We should be concerned lest the Ukaf or wallet or ring was borrowed!

Answer: One does not borrow another's Ukaf, lest it wound his donkey. One does not lend his (signet) ring, lest the borrower forge with it. One does not lend his wallet, lest it harm his Mazel.

Answer #2 (regarding clothing): The Simanim on his clothes are (vague and unreliable, e.g.) they are white or black.

Gitin 27a (Mishnah #1): If a man (a Shali'ach) was carrying a Get and lost it and found it immediately, it is Kosher (we assume that it is the same one). If not, it is invalid.

Contradiction (Mishnah #2): If one found a Get or gift document... he should not return it to the recipient. Perhaps the giver reconsidered and never gave it.

Inference: If the giver says to return it, we do, even if it was found long after it was lost!

Answer (Rav Ashi): The case is, he (the Shali'ach) tells us that there is a hole near a certain letter in the Get, which is an exceptional Siman.

Inference: Had he said only that it has a hole, which is a mediocre Siman, we would not return it (if it was found later).

Rav Ashi is unsure if the Torah authorizes returning an Aveidah based on a (mediocre) Siman, or if this is only (a monetary enactment) mid'Rabanan.

Rishonim

Rif and Rosh (Yevamos 45a and 16:3): All agree that Simanim are mid'Rabanan. They argue about whether or not a wart is an exceptional Siman.

Nimukei Yosef (DH Gemara): We hold that one may rely on an exceptional Siman on the Get or the Keli in which it was found.

Rif: We return a Get if the Shali'ach tells us that there is a hole near a certain letter, which is an exceptional Siman.

Rambam (Hilchos Gerushin 3:9): If a Get was lost in a place where caravans are frequent, if it was found in the Keli in which he left it and he recognizes the length and width of the Get when it is rolled up, it is Muchzak to be the Get that was lost, and it may be used.

Rebuttal (Ran 13b DH Masnisin): Also Rashi requires that he recognize the Get and the Keli. This is unreasonable. It suffices to have a Siman in the Keli! We return a donkey to one who gives a Siman on the saddle (Bava Metzi'a 27a)! Also, it suffices to recognize the Get. Even an Am ha'Aretz has Tevi'as Ayin (recognition), and it is better than Simanim. We kill people based on it. Men are permitted to their wives in the dark based on recognition of voice! We do not trust an Am ha'Aretz regarding a Metzi'ah, for perhaps he is lying. When he himself found the Get, he has no need to lie. He did not need to tell us that he lost it!

Beis Yosef (EH 132 DH v'Amai): Our text of Rashi says that it suffices to have a Siman in the Get or the Keli.

Rambam (11): If the witnesses had an exceptional Siman on the Get, it is Muchzak to be the Get that was lost, and it may be used.

Rosh (ibid.): Rashi's text says that the Shali'ach gives the Siman before seeing the Get found. If the text says that witnesses give the Siman, it is even if they do so after seeing the Get.

Tosfos (Gitin 28a DH Motz'o): Some say that 'if he recognizes it' refers to the Get. Even if it was found in someone else's pouch, it is Kosher if he recognizes it. Alternatively, even if he knows that it is his pouch it is Kosher only if he recognizes the Get.

Poskim

Shulchan Aruch (132:4): If one lost a Get it in a place where caravans are frequent or the city is Muchzak to have two couples whose names are exactly those on the Get, even if it was found much later it is Kosher if he has an exceptional Siman on the Get or recognizes it. Alternatively, it is Kosher if he left the Keli in a Keli and he has a Siman on the Keli or recognizes it and knows that he did not lend it to someone else, or it was a tied to Kelim that he does not lend, i.e. a money-bag or ring.

Beis Yosef (DH v'Chivan): Since we are unsure whether or not Simanim work mid'Oraisa, if a Get returned with a mediocre Siman was used she is Safek Megureshes.

Rebuttal (Darchei Moshe 2): Even if it was returned without any Siman she is Safek Megureshes, for perhaps it is the lost Get!

Shulchan Aruch (ibid). If the witnesses testify that they never signed on another Get with the husband's name, and we know that there are others with the same names as the witnesses, the witnesses must testify that these are their signatures or give an exceptional Siman on the Get. If the one claiming (that he lost) the Get gives the Siman, it helps only if this was before he sees the Get that was found.