We have two species of crow here in Florida, and they're nearly identical. There are differences between them, but it's very difficult to tell them apart by sight alone. One author writes that after studying these crows for thousands of hours, he still can visually distinguish them reliably 80% of the time. By far the most reliable way to tell them apart is by voice. American Crows "caw," while Fish Crows give a nasal "uh-uh" sound. Fish Crows are larger than American Crows, but unless I see them together, there's no way I could tell one from the other by size. But there are a couple other hints that can sometimes be helpful in telling them apart.
Fish Crow at Fort De Soto
Fish Crows are more common than American Crows throughout Central Florida, in some places American Crows are generally absent, especially in coastal areas like Merritt Island and Fort De Soto. In these areas, if you see a crow, there's a pretty good chance it's a Fish Crow. The above photo was taken at Fort De Soto, so even though I never heard it call, I'm still pretty sure it's a Fish Crow.
I took the above photo of a calling crow on Lake Monroe, where you might expect to find both species, but I was far enough behind in my editing that I forgot which call I heard when I took it, and my eBird checklist included both crow species. But Fish Crows extend their throat feathers when calling, while American Crows do not. So thankfully I was able to ID this crow.
Fish Crows are generally shorter-legged than American Crows, but I don't feel confident about this field mark at all. Sometimes I think when I'm looking at a Fish Crow, I can tell that the legs are shorter than an American Crow, as in the photo above. But I don't think this should be used as a sole guide.