I hate to say this but when you hear a system that really is fast you'll know it when you hear it.
Things to look for: good decay (lots of it- good lower level detail), nice sound stage that is expansive rather than stuck as a little window right in the middle of the speakers (that 'wall to wall' soundstage can only occur if you have the bandwidth to prevent phase shift in the upper portions of the audio range), 'blackness' in the silent spots, but things like cymbals and strings have an effortless, relaxed quality...
Brightness is usually **not** an indication of speed (brightness is either a tonal coloration or the product of distortion); in fact a relaxed quality is a much better indication. Relaxed (even with the volume turned up!) yet excellent detail in the highs. The two go hand in hand if you have a fast system. Another thing- you might react more viscerally to the sound when there is speed. If you are tending to analyze the sound then its probably not fast.
