If you have the abillity to play around with your resolutions on your monitor one simple way is to see what you are getting in a game at a high res, Run Fraps to check your FPS.
If you now drop the res to say 800x600 then your FPS should increase if they dont then you are being bottlenecked as what happens is that the CPU does a lot of the physics ie where stuff is on screen when stuff is moving about.
When the CPU cant keep up with the amount of info the GPU is throwing at it you get the bottlenecking effect.
This is a very simple test that may or may not work it all depends on how hard you can work the GPU in the first place.
Start max ress AA etc and then drop the res if the FPS dosent go up then the CPU isnt keeping up.