I think that it sounds more natural to say
"Are you on staff?"
Or, as @MattGibson points out, since staff is a collective noun it also sounds natural to say
Are you on the staff?
If you ask, "Are you a staff?" then it sounds as if you are asking whether the person is a long stick of wood. You could, however, say "Are you staff?" This is pretty informal, though.
If you want to make your question more specific by asking whether they are a member of the ABC staff, you can ask:
Are you on ABC staff?
Are you an ABC staff member?
Are you ABC staff?
The last is pretty informal, but should be understood by most native English speakers. It may not sound correct to their ear, though. To be on the safe side, it is best to add a preposition (are you on staff) or to fully clarify that the noun is staff member.