Many US Buffalo nickels in the teens and twenties, struck at both the Denver Mint and the San Francisco Mint, come wealy struck from worn dies.  You have to know how an issue should look when in mint state to
properly evaluate the circulated grade.  As the dies used to strike these coins on nickel planchets were very hard, often times the detailing can be
weak.  The issued from Philadelphia tend to be well struck.  You can see
D and S Buffalo nickels with a mushy, weak strike on the Buffalo's head on the reverse that are Mint State.  On the converse, you can see a circulated Buffalo with a full horn and full Buffalo head that are only Extremely Fine.
Study the years and mintmarks by looking at coins offered on eBay to self-educate yourself.  David Lange has written an excellent reference guide for Buffalo Nickels.