Good question. I'm not so sure that focusing so intensely upon a couple of players is the best approach--unless they are indeed the only players at the table that you're not already familiar with.
My guess is that employing more of a "soft focus" approach might be more productive. That is, rather than looking for so much detail on a few players, try to take a broader view of the table and become more aware of out-of-the-ordinary actions that arise.
If you haven't seen
this , take a look and see if the same sort of phenomenon might be happening to you.