How do you tell the difference between fruit bat or vampire bat?

Bring a UV flashlite and shine it on the bat. If it's a vampire bat it will burn up.

Also, fruit bats have heads that are kindof like fox heads, with dog-like noses and ears, where vampire bats have the upturned nose and their ears are more like holes in their head. Fruit bat wings are sometimes less hairy than vampire bat wings.