A MOSFET should show conduction like a diode in one direction from Source to Drain. A shorted one will be a short in both directions, and this can be measured in-circuit. However, if there are transistors in parallel both/all will appear to be shorted. The positive output of the bridge rectifier will connect to the positive terminals of both 680uF capacitors, and then go on to the inverter section. If you cut the track just after the capacitors, that may allow you to power-up and check for the 325v DC.