How do you differ them?

the top view of the snout is the easiest thing you can see! Atractosteus have snout a lot thicker than lepisosteus.

With cubans and tropicals being so rare and expensive, I doubt you'd be staring at an unlabelled atractosteus gar that isn't a gator.

That in mind, L. gars are silvery with the black stripe down the side, whereas all the baby gators I've seen look like they're wearing frog skin, Buffalo Bill style.

IIRC A. gars have eyes almost the diameter of their faces.

read the skicky on the different species.. there are picture of juvies and adults of all of them..