Use PokerTracker and if your stats show that a player rarely raises preflop, and 3bets preflop even more rarely, then you can safely assume that if he takes a stand against you, your JJ and probably QQ is beat. Conversely, if a player who raises preflop every time he enters a hand, and plays a high percentage of hands, I"m more than happy to get my stack in with JJ (and worse)