I have a 4 blade a/c transformer. How do you check to see if it is good? It is already off.

Is this off a HVAC circuit board of furnace?
If so model number and brand

Check the side of the transformer as it may show which side is the 120VAC input and which side is the output.

If it doesn't say you can trace the input wires back through the furnace. Once you know which side is the output (I'm guessing 24VAC) just wiggle back the connectors and put your probes on that side.

Don't forget you probably have a safety cutoff switch on your furnace that cuts power when the panel is removed. You'll need to press that in to energize the transformer's input.

If the transfomer is working at all they often times hum a little and build up some heat. That doesn't mean, however, its outputing the correct voltage, just that its not dead.