[soon2bepro]

There's no solution to this. (unless there's some information you missed to provide)

There are only 4 possible settings for the two switches:
1) A up, B up
2) A up, B down
3) A down, B up
4) A down, B down

So there's no way for you to know just by getting in the room that 29 other people have been there, since there are only 4 possible settings and you'd need 29 at the very least (possibly more, since you have to take into account that whenever you get there you must move a switch even if you've been there before).

In your hint, nothing changes much, you have 2 possible settings with which you need to account for 29 other people. No way.

I guess the group strategy would be to wait a couple of months then say everyone has been there. Worth the risk.