Grading 1921 Peace Dollars poses a different set of grading eyes, because this coin was struck in high relief, and will more often than not display heavy flatness on the hairline along the back of Liberty's face. This flatness isn't wear and other areas of the design must be considered to properly grade a 1921 Peace Dollar in AU-MS grades. Concentrate on the part of the hair just above the forehead and the hair bun, on the obverse, to look for wear and look at the high point of the eagle's wing on the reverse.
Disclaimer: This is just a guide to give you an idea of what your coin might grade. Often coins will grade higher on one side vs. the other. In example, it's possible for a coin to grade G4 on its obverse, but grade VG8 on its reverse. Also, this guide is not a claim that one's coin will obtain any grade listed if submitted to a third party grading service.