What should I test first on this PSU board that I can test in circuit There's some dried brown stuff all over the place Is that discharge from a capacitor?
It looks like factory goo to absorb vibration and simply glue components. Given the colour codes of the wires, it's obviously a computer PSU.
Completely dead?. Check the input voltages, cables and bridge rectifier.
It is always possible that a diode just failed short, but not likely.
Remove the diodes or bridge IC and check to see if a short on the PCB is still present.
If it still has a short, check the switching transistor and capacitors.