How do you measure the volt-ampere rating of a transformer?

No need to measure, on the side of every transformer is a nameplate with all the transformer's specifications. Primary and secondary voltages, impedance and winding ratiOS. Need more info. What are you trying to do? <><><> If it is a small transformer - such as a one that is soldered onto a printed circuit board in a radio or a computer power supply- then it may not have any nameplate. If it is that type of transformer it will have been designed to do its job and you would not need to be able to measure its volt-amp rating. If the transformer is of the larger type that runs on household mains voltage that was mounted separately inside some equipment - such as an old battery charger, television, etc. - then you would have to use some suitable lab test equipment to be able to apply a variable load to its secondary winding (or to the appropriate tap on an auto-transformer) whilst measuring its output voltage and body temperature.

Such tests should never be attempted unless you really know what you are doing and have the right test equipment to be able to do it safely. <><><> As always, if you are in doubt about what to do, the best advice anyone should give you is to call a licensed electrician to advise what work is needed.

Before you do any work yourself,

on electrical circuits, equipment or appliances,

always use a test meter to ensure the circuit is, in fact, de-energized.

IF YOU ARE NOT ALREADY SURE YOU CAN DO THIS JOB

SAFELY AND COMPETENTLY

REFER THIS WORK TO QUALIFIED PROFESSIONALS.