[ramjam]

If this is a loose low-limit game where any ace is played but nobody ever raises (even with AA or AK), you can solve the problem analytically.

There are 47 unknown cards, your opponents were dealt 18 of them and there are two aces unaccounted for.

The probability that those 18 cards contain no ace is 28*29/(47*46)= 37.6% (I think!). Or put another way, there's a 62.4% chance that someone else does have an ace.

As nottom says, if your opponents are more selective in what they play, you can't solve the problem without knowing exactly what their calling/raising standards are in every situation - and even then, it would probably be too complex to solve this millennium.