Resistance ....

was the wrong word to use in this context.

NOT Electrical resistance but PHYSICAL resistance.

Can safely measure voltage in parallel to the tested circuit.. so can validate voltage without problem..

Current is the risky bit.. depending on amps coming from the supply... but current generally equates to power or torque in motors and this can safely and easily be tested with a bit of physical resistance to the motion of the motor or whatever it's driving...

I.E. If you get good speed but you can stop the motion with light pressure it's likely that voltage is OK but current is Insufficient. If you get great torque but no speed then that would suggest current is OK and voltage is down. It's all fairly simple stuff really...