I check for low resistance between PROM socket pins with a multimeter.
Apart from a couple of pins that a re 30-40k ohm the rest are in the megs. I have found the a couple with low resistance between the pins that clear by heating the affected pin and allowing the solder to flow.
I have had a non booting board that was caused by a solder splatter away from the PROM. A brush with a bristle paintbrush (not nylon) with the board upside down should clear these as they usually aren't well stuck on.