R. Yonah: Man's Midos and nature are straight in his eyes. He does not recognize their falsity or crookedness. Therefore, it is very hard for man to fix his natures and Midos. Hashem helps man to do so, and arouse his Ru'ach to sense its lack and crooked Midos. When man directs his heart to seek the Emes and follow Hashem, Hashem sees the heart - "Ki Chol Levavos Doresh Hashem v'Chol Yetzer Machashavos Mevin" (Divrei ha'Yamim I, 28:9); 'one who comes to purify himself, [Shamayim] helps him' (Yuma 38b).

Malbim: Zach is the opposite of Pesoles, and Yashar is the opposite of Avel. Yashar is in Binah. Derech ha'Yashar is the shortest path between two points. E.g. the way of humility, generosity and mercy is Derech ha'Yashar. Sometimes one must act oppositely, e.g. haughtiness and cruelty against those who 'afflict' Hashem. Man errs about this based on his heart's leaning. It seems that this is straight, because his heart and choice lean to this. Hashem is Tochen Libos. He knows the heart's nature and weighs its paths. Some do not err, and go straight, but it is not Zach (clear), i.e. he does for external motivations. This depends on the Ru'ach, which puts images on the heart. The Ru'ach is deeper than the heart. Therefore, it says there "v'Sochen Ruchos Hashem."

Rashi: He counts.

R. Yonah: Sochen is an expression of fixing, like "Yikon Derachai" (Tehilim 119:5). This is connected to the previous verse. Even though Hashem gave to man choice about deeds - "Re'eh Nasati Lefanecha ha'Yom Es ha'Chayim v'Es ha'Tov" (Devarim 30:15). However, Hashem tilts the king's heart to whatever He wants, and switches bad natures and fixes hearts, when one directs his heart to follow Hashem and serve Him.

Malbim: He knows the heart's nature and weighs its paths.