Well, that can be kind of tricky. I can't tell you exactly where to look, but I would first try rooting around the control panel area. Check anything that says system configuration or hardware profile. If push comes to shove, you could open the case and physically examine the RAM modules to see if they list what type and brand they are.
BTW, what model HP do you have?
UPDATE: Don't know how I could have been so stupid, forgetting crucial.com. Try http://www.crucial.com/
, use the little tool there to see if you can't find your model.