Measuring the specifications of a DC motor of unknown torque/power.

Measuring RPM, then Measuring Torque to create a torque/speed curve for the motor.

DC motors are pretty much linear , the faster they turn, the less torque they have. Max torque occurs at stall (as does maximum current)

Drawing the torquespeed curve can be done by measuring the noload top speed and then measuring the stall torque. Now any point along the graph can give you an estimate of the motor's speed and the torque at that speed.

The torque speed "curve" can also give you an estimate on how much current draw the motor will require.

You can caculate the actual mechanical output power of the motor by using the RPM and the torque together.

Be aware that maximum EFFICIENCY, where the electrical energy is converted maximally to mechanical energy, isn't at the same point of maximum motor mechanical power. Typically max efficiency is around 75% of motor's no load speed.

This particular motor could no doubt put out up to even 140watts with higher voltages.

The best way to measure would be to use a dynamometer, or a "ponybrake", but I have not built one of those yet, so doing it this way will work to give you an estimate of what the motor can do