It's gets trickier to judge the more cores the cpu has - and since the majority of games don't know cores, when something like a quad is hitting 25%, that means the game is struggling. i.e. 1 core at max. Even the ones who claim they're core-friendly seem to utilize duals and not quads.
The easiest people to speak to are those using things like G15 keyboards. If they're like me, I keep the performance tab on all the time. Games don't generally use more than 30-40% of 4g, 20% of 8g is currently being used playing Reckoning. The cpu meter is easy to check if a quad is hitting 25% and you can see 1 solid line. i.e. one core maxxed.
Biggest shocker I had was watching Crysis 2 run on a E8400 (around 3.8ghz) - both the cores on the G15 were showing full at all times. After that, tried it on a qx9650 at around 3.6ghz and it'd show 25/26% cycling through which core it maxxed.
Complicated thing to test. The G15's (or any external LCD utility monitor) are perfect for this tho.