A few things:
<ul type="square">[*] What will this player raise preflop with?[*] What will this player 3-bet preflop with?[*] Does this player seem positionally aware?[*] Does this player ever bet a draw or bluff?[*] Does this player call down with any part of the board (or a pocket pair)?[*] Is this player worth trying to isolate with marginal hands?[*] What will this player check/raise with?[*] With what hands will this player raise against a dangerous board?[/list]
Hope those are some good ideas.