As Nacho says, its usually stated, you can get a guess from the electrical ratings, and you can, if you are absolutely forced to, do it by measurement. If you know the torque, and the speed it spins at, then Torque in Newton-metres x speed in revs/second gives power in Watts. Divide by ~750 for Hp. Steve
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Note too that "horsepower" originated as a marketing term and has very little to do with the amount of work an actual horse can perform.
. I just look at the nameplate - it's usually one of the first params listed.