Here's a trick: think about the game, your opponents, the betting, recent hands, and the stacks. Then make a wild-ass guess.

In general, absent reads, I'm willing to stack off with any set or flush and nearly any straight (it's easier to tell if you're beat with straights, typically). I'm willing to stack off most times with 2 pair (70% maybe) . I'm often willing to stack off with TPTK or an overpair that's QQ or better (maybe 30%).

The average winning hand in HoldEm is two pair. If you've got something better than that, you need to be willing to felt it.