Originally Posted by PeteB
well I have a voltmeter so I can measure all sorts of stuff, but I thought that I would need to measure the output AMPs, not volts...
You said yourself that Power = Volts x Amps
Anyways, another method be a calculated flywheel method. Attach the motor to a flywheel of a given mass and you can derive the force needed to spin up the flywheel in respect to time. Then you can calculate power.
You can also use a cheap photo-tach sensor (20 bucks from Towerhobbies) to measure the RPM under load.