you could always say you have some other offers on the table from other universities and you need to get an answer asap to make a decision. as has a already been said the worst that can happen is that they tell you to wait for the official response. But i think most supervisors are reasonable in this respect and if you send them a polite email to that effect they will probably give you the verdict if they have already made their decision. it might not be finalized with the university but university admissions department will 99% of the time accept a student on the recommendation of the supervisor you have applied to, should they want to take you on.
hope it works out for you
rjb