I have a old but very nice sounding SWR Workingman's 12 amp, which I bought a while back for my son to practice the bass guitar on.  It's the older one with the controls set back at the top.   All seems to work very well, except for the clipping light, which doesn't seem to do anything regardless of any settings.

I was wondering if maybe his cheap Bass guitar's passive output is a bit weak, or is it the clipping light that's faulty. 

Can anyone give me some advice on how to test this please.