there is of course a couple of ways, one could go about this....
1. is of course the AMP meter, in line between Battery and Motor, then try and stop the motor by clasping the drive axle, with a pair of pliers.... if you can stop the motor, its not very powerfull in the sense of Hose power, take a look at the AMP meter while your trying to stop the motor, and you will see it rise, if you manage to stop the motor, then what is shown on the AMP meter, is the maximum Drain...
2. the other way would be to put a known Ampage battery, directly onto the motors, while the motors are under strain... (boat in the bath or other water), and see how long it takes to drain the batteries... if a 4 Amp battery is empty in 15 Min's, then the motors are drawing about 16 Amps
hope this might help a little