Connect an adjustable power supply to the windings with an amperemeter in series. Then increase the current in steps, waiting half an hour between each increase. When you cannot touch the motor (around 60C) you have an approximate of the max sustained current. Measure the voltage over the windings, and you have an approximate voltage rating. Measure the torque you can apply to it to cog over, that's the holding torque. Use the pulley, rope and scales method.
You can run up to 4 times as much current through it for a short time on a modern motor. For old motors, I'd relax this to 2 times the current.