It is damn near impossible to tell (and definitely is in this case). That's why you just have to run with it. He could have limped Aces, have an overpair TT-QQ, or he could have 99 or 77 - you'll just have to pay him off in that case.

Now - if you had raised to $10-12 on the flop, and had then been reraised allin or something, you MIGHT be able to get away from it...even then, I don't think I've read of but one case of someone folding set over set around here, and it wasn't in a case like this.
I know I never have and I just live with it...