You have to determine whether an opponent is balancing their lines effectively. Sometimes an opponent will bet half pot with a monster, get raised and push over for a fold, and feel this is sufficient enough to balance their weaksauce feel bets for the future.

Bottom line is that it's a math game. If someone bets weak when weak too often you keep stealing from them, and let them have one here and there. If someone bets weak when strong too often play drawing hands in position against them. IE call with more suited connectors for deep enough stacks.