I think it would be impossible for them to know. The way I see it is the March 1st or whatever (can't remember date) deadline for the second deposit payment was when they'd have their first group of spots that would open up, but then they may have filled those with late interviewers or just let those spots accommodate their initial overfilling. Then the deadline for committing to one school, May 15th or whatever it is, would be when their second group of spots would open up and they'd fill with waitlist spots once they've had enough people drop from the assumed initial overfilling.
So, basically, somewhere shortly after May 15th, you'd expect to see multiple people come off the waitlist. Then from that point forward, the only movement would be from the sort of waitlist cascade amongst all programs, where someone gets off the waitlist somewhere, takes that spot, opening up their current spot, and then that spot now needs to be filled. So spots can theoretically open up all the way until classes start, but it would require a current accepted student to get off the waitlist elsewhere and thus forfeit their spot. It wouldn't make sense to me that this process would occur any other way, and that goes for all medical schools. I say that because med schools have all finished interviews a while ago. So they have known their current pool of applicants for plenty long enough and wouldn't be holding spots open for no reason or taking this long to make decisions on a group of spots.