While alligators and crocodiles share a lot of similarities and are often confused, there's an easy way to tell them apart.
One generally reliable rule is that alligators have U-shaped heads, while crocodiles are V-shaped - which can be remembered by noting that "A" in alligator comes before "C" in crocodile, and "U" comes before "V." If you're still confused, try to get a look at their mouth. In alligators only the upper teeth can be seen with the jaws closed. In crocodiles upper and lower teeth can be seen.