I believe the question is actually how to test the 12X4 rectifier tube.
That is more difficult, unless you have a variac.
The 12X4 is probably the most reliable component in the PAS3.
I've never in 30 years seen one go bad, so I think probably the only thing that would cause it
to go bad, is a bad or shorted quad capacitor, or bad or shorted filament supply.
I would replace the quad capacitor with a new one (about $40), or much better, with a new power supply board (about $50),
that replaces both the quad cap and the dual filament supply capacitors and diode rectifier with one all-in-one solution.