1) make a dynamometer. It isn't hard to build a force balance, and once you've done that everything else is easy
or
2) add a balanced rotor of known inertia and do a dynamic spin-up test
3) drive a known generator from your unknown motor. There may even be some handy reciprocity laws about using two identical motors as a motor-generator set.
How small is your motor? Very small motors can be tested using an eddy current dynamometer. A cylindrical magnet is spun by the motor inside a stationary metal cylinder. The radial clearance between the magnet and cylinder must be small. The metal cylinder can be any conductive metal, and is easiest to work with if it is NOT steel. The metal cylinder is attached to a small beam that weights are hung on to counteract and measure the torque. The resistance is varied with motor speed and how far the cylindrical magnet is placed inside the cylinder. This device will give your torque very accurately, but speed must be measured by another device.
Small motors like the speed 400 are very difficult to evaluate. You need a device to measure rpm. You need a device to measure torque.
To measure torque, build a device that cradle-mounts your motor/prop with a ruler mounted on it. A known weight is arranged so it can slide along the ruler. Before powering up, the cradle/motor/ruler should be perfectly balanced. Then run the motor. Slide the weight until the rig is again perfectly balanced. Read moment arm from the ruler. Multiply by the weight to get torque. Read rpm. Multiply by torque to get power.
