While not perfect, a good approximation for estimating a motor's maximum horsepower is to use 0.8 for powerfactor and 0.8 for efficiency, and base the calculation on the motor's nameplate current value. If you really want to push a motor to its maximum, use 0.85 for either, or both.
HP = I x V x pf x eff / 746
A motor's hp rating is not an absolute, but depends on how far the manufacturer is willing to push the motor into its thermo-electric curve while still maintaining a warranty. In other words, you can get more power out of the motor, but it may damage the windings in the process.
Oops, sorry I forgot the 3-phase conversion. For a 3-phase motor:
HP = I x V x 1.73 x pf x eff / 746
Alright, a second edit: based on your numbers, a 15 amp motor on 208 V 3-phase, this motor is between 4.6 and 5 hp.