How can I be sure that your software will actually use my GPU?

Trial version uses GPU if it's possible, so if it doesn't -- your GPU is not supported.

Basically you're need NVIDIA G80+ (8600GT and above) or ATI RV710+ (4550 and above) video card to get GPU acceleration.

You can open GPU Configuration dialog box (Menu Options->Preferences; Tab GPU Configuration) to check out which GPUs has been detected as supported ones.