Clock speed, GPU load and GPU temperature are usually good indicators that a GPU is performing up to standard.
I suggest you download GPU-Z and FRAPS.
Turn on FRAPS and GPU-Z before running a games that has good performance scaling and see what frame rate you're getting, exist the game after a couple of minutes of playing and have a look at your GPU temperature and load. If only 1 GPU shows activity then you know that the crossfire is not working.
If both are showing activity and you simply want to know the performance boost. Disable crossfire and go into the same game again (still with GPU-Z and FRAPS on) and see if there is a noticeable drop in frame rate.
Good games to use for this test would be Metro 2033, Shogun 2, Witcher 2 or any game that can scale enough when you increase their graphic setting to stress high end GPUs.
