Other than the fact that you have to expect the answer to be “no” or silence ... :-) In US English, and I believe elsewhere, these mean exactly the same thing. You could say “asleep” describes the state and “are sleeping” the action, but that’s a distinction without a difference. Is he asleep? is exactly the same question as Is he sleeping?. You’d never say “yes” to one and not to the other.

Asleep is an adjective in Are you asleep? while sleeping is a verb in Are you sleeping? Use asleep if you want to emphasize the state and sleeping if you want to emphasize the action. Basically, it tells you that you can use both options and it doesn´t really matter which one as a regular basis. If you use asleep, you are interested in how the person is, asleep and not awaken. "Its 11 am and you are asleep, really?". In the other case, you are more concerned about the action. "Sorry, are you sleeping?". But as you can guess, if you use change both of them, it´s perfectly fine.