When a new person joins a table you are playing, how much respect do you accord his/her raises?
The First Time: I will give him the benefit of the doubt.

depending on teh outcome/his cards

then the Second Time: Call, but fold if you flopped nothing.

depending on the outcome/his cards

then the third time (all within a reasonable amount of time): more likely to reraise and put him on the defensive.

Sometimes players will bet as long as no one raises them, then once someone raises them they check/fold. They do this in any position and its relatively easy to spot.