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The easy way to test if it's being stressed is to add more detail to the windows and see if your frames drop. If they don't, you were CPU limited in the previous settings/resolution. If they do, you're pushing your GPU as fast as it will go. That's how you know if you're being limited while reviewing hardware - just check the fps/performance charts of videocard reviews.

If a chart has 4 resolutions: 1024, 1280, 1680, 1920.. and the fps curve looks like this:
145fps, 145fps, 128fps, 64fps..
Then you're CPU limited at 1024x768, and GPU limited above that.. with the possibility of GPU limit at 1280.
That's why a lot of high-end cards have fps curves like this without AA enabled:
150fps, 148fps, 148fps, 147fps.. if it's flat, the GPU isn't what's holding back performance.