Rashi: Hashem weighs them, and knows how many sins and merits he has.

Malbim: Derech is the straight path. "Magal" is a roundabout path. Measuring the roundabout path needed to get to the straight path is called Pales. The straight paths of one's Nefesh are "Nochach Einei Hashem." He prepared the straight path via the Torah - "v'Hodata Lahem Es ha'Derech Yelchu Vah" (Shemos 18:20). He also weighed all the roundabout paths. If sometimes one must go on a Magal that turns away from the straight path, e.g. Midas Chasidus and beyond the letter of the law, or to transgress Torah law according to the need of the time, e.g. Eliyahu on Har ha'Karmel. Hashem directs the Magal, so via it one will reach the straight path The Midah used to gauge the roundabout path needed to get to the straight path is called Pales.