Perhaps I am naiive, but I find it hard to believe that other offers might affect your offer from your first choice. Surely the university has a website where they list "selection criteria", and whatever it says there should be what matters. I doubt other offers would be listed on such web site. Then again, I said I might be naiive with this, perhaps universities are not as honest as I would like to think that they are. In fact I am pretty sure they are not. But to answer our question, that's a tough one... Perhaps write a little vague email saying you have a firm offer from this and that university and a couple of other ones pending or under discussion, but that it's ok and you're willing to wait.
(I am an international student from Europe so my apologies if my answer seems completely irrelevant or ridiculous).
Good luck with your first choice application!