Rodentia is the largest order of Mammals, with nearly 2,000 species worldwide (about 40% of all the living mammals). Most rodents have 4 toes on the front foot and five on the hind foot. All rodents have similar teeth: one pair of upper and one pair of lower incisors, then a large gap and several pairs of chewing teeth. This incisor teeth grow continuously throughout life and have chisel-like cutting edges. (Peterson’s Guide to Mammals of North America, page 177).
Here is a neat way to use a quarter to determine chipmunk vs red squirrel vs grey squirrel, shared by Chris Earley – an amazing naturalist and author of several nature guides: “I use the quarter to get a sense of the size of the tracks, plus I can extrapolate it for measuring if needed. In general, I find the hind foot track of a chipmunk to be smaller than the quarter, the red squirrel to be about the same width as the quarter and a gray squirrel to be bigger than the quarter. This is for snow tracks – in sand the tracks may be more spread out.”