Veiltails have no ray branching in their tial. Plakats rays branch at least once, and their tails are more rounded. That's what it says in the pinned topic "Betta tail types" anyway.
Actually, some veiltails do have one branch. It says that in the topic too, I wrote it. However, if you have a female with no ray branching, you do know for sure it's a veiltail.