The only thing you can do is ring it out methodically with a multi-meter.
Checking continuity (or not) from each wheel on each side in turn to key points on the chassis and pickups towards the motor.
Then check for any cross continuity side to side noting that if you jiggle the valve gear or motion rods these can touch against a wheel rim and possibly produce a short. Mount the loco upside down in a cradle - I made a simple U-shaped one out of polystyrene foam offcuts.
It just a case of going through the loco power path end to end looking for the short, which could be as simple as a frayed wire rubbing on metal somewhere. It may be that you have to disconnect the motor wires one brush at a time to eliminate that from the problem as the motor could be shorting internally.