There's no given system. Some feats might be obvious if used successfully (Cleave, Improved Trip) . Also, as a DM I usually provide a little clue in the descriptions, but not too much. For example, I might say, "As he attacks, you notice that he seems to be holding back, concentrating on not leaving himself open to a counterattack". The NPC could be using Expertise, or could just be fighting defensively. Or "he seems to be swinging a little recklessly" (Power Attack?).