。
Have you made a reservation?
Literal
Reservation [topic-は] do-[resulting-state-てある]-[polite-ますか]?
~てある differs from ~ている in subtle but important ways: ~てある marks a state that resulted from a deliberate prior action, usually with the doer implicit. 予約はしてあります = 'a reservation has been made [by someone, for this purpose].' Compare ~ている, which marks ongoing action or a resulting state without the 'deliberately done for a purpose' nuance.