a HUD is much better for that, but it costs money. If you don't have a HUD, then it is useful to take notes, in case you see the opponent's cards, to see how he played. But the thing about notes is that they are valid only during that session, not in another. For example, you are playing a tournament and you take notes on players. The mext day you play cash and you find a player who has a note from the yestarday, they are not reliable, at least it is what I think