[murphyv310]

Sometimes you can.
Use a high impedance meter, preferably a VVM, one lead on the Chassis and poke around the board its self not the contacts, any voltages picked up will indicate a leak. Also worth looking at this on a scope, it might surprise you.
Ken on the forum sent me a Pye radio last year with an obscure issue, the problem was the valve base on the FC, it was leaking from the heater connections, a new base effected a cure.