Only real way to check the power is to fire a pellet through a chronograph and then you can work it out.
Your local gun shop (if you can find one) should be able to help you out.
I bet that costs a bit. What sort of time frame does it take from shot to hit (noise) from about 50 ft? (I know it's not vastly scientific but may be a bit of a pointer). Instant?
EDIT: Ah cool, although the old local (if you can call a half hour drive local) shop has now shut down, but I'm sure I can find another one. Would they be likely to have a chronograph or it a special piece of kit that not many would have?