As far as gameing is concerned, some games can actually show CPU and GPU bottlenecks from one moment to the next. A good example would be crysis, where you can get a large scene bottlenecked on your GPU, and then a big explosion causes the frame rate to drop as the system becomes CPU bottlenecked trying to perform the physics required for the explosion.
Basically...
If a game is running at the same frame rate constantly, and increasing resolution does not significantly slow it down, the game is probably CPU bottlenecked.
If a game is running at the same rate constantly, and increasing resolution significantly reduces frame rate, it's probably GPU bottlenecked.
If the game swings wildy in frame rates, it's probably both.