There are several useful physical features that will help you tell the difference between grizzly and black bears. In general, none should not be used alone. Color and size are not reliable indicators and should not be used to identify species.
The training pages, at the links below, will work through each of these characteristics one by one, but here is a general graphic look at all of the useful features. In many cases, it's not possible to identify a bear by using just one feature, therefore, take your time and look for as many features as possible to help ensure you make the right call.