Comb differences: one flops over, or has bigger or smaller points, or something like that.
Color and pattern differences: one is a brighter or darker red than the other, has a different amount of red in the breast, has sharper barring or more/less of it.
Anything a little bit different--a crooked tail, extra-fluffy feathers, tail held high or low, walks proudly or timidly, the first to run to you or away from you, etc.
Leg bands are a great idea.
I have been known to cut the tail feathers short on one chicken, and leave another of the same breed untrimmed--easy to tell them apart, at least until the next molt.
If you really like being able to tell your chickens apart, it's easier if you get only one of a breed, or if you get breeds with patterns in their feathers: they seem to be more variable. For example, my sister has two Golden Laced Wyandottes where one is nearly black and the other has lots of gold.