If you have PokerTracker and some kind of a Hud, when someone PFRs or limps in, look at thier stats. Here's some common ones (VP$IP/PFR%/T.Aggression):

A rock (Tight-Passive) with a (17/2/0.7) puts in for a PFR UTG. What do you put him on? He's likely got either AA, KK, or poasibly QQ or AK. Unless I have one of those hands, I insta-fold.

A loose-passive player with a (40/1/0.5) all of the sudden bets on the turn with a possible flush on the board. Unless you already have a A (or maybe K) high flush or better, insta-fold.

A losse-aggressive with a 45/20/3.5 raises from CO with no prior limpers and you hold a PP, Insta-Push. He's likely trying to steal the blinds and holds weak cards.

Base it on what they limp and PFR with, bet on and thier TA factor, and on the obsevations you've made with this individual or those classified like him. Think ranges, not actual hand until the River. By then you should have a good read before going to showdown.