In southern New England, we have two types of rats: Norway (brown) and roof (black) rats. For mice, we have deer mice, house mice, and white-footed mice.
If you see something scurry across your garage floor, it is hard to tell what it is. Especially since you see them infrequently, and you only see one of a species.
General differences begin with body size. Mice usually weigh 0.5 ounces. While rats are 7-11 ounces.
Rats have bigger hairless tails that are as long as their bodies. Mice have hairy tails that are shorter than their body length.
Mice have big ears when compared to their body size. Norway rats do not. Roof rats also have big ears.
The last key difference is in the snout. Mice have triangular snouts; rats are more blunt.