A friend of mine (who is also a fellow VOGONer BTW), has CT1600 which is in very nice visual condition (no scratches etc, other than two dented caps which shouldn't cause the described problem). A
Unfortunatelly card is not detected by the system - like it's not there. We compared it against mine CT1600 which works fine (tested in the same machine, same jumper settings).
Up-close inspection didn't revealed any cold solder joints under the smd chips (it was a cause of similar behaviour in one of mine CT1350Bs - cold solder joints under the smaller intel chip).
Friend of mine suggested to test the ISA goldfingers with the multimeter on both cards to see if there are any differences, but I'm not sure about it (I think it will hardly prove anything and actually may cause some damage - correct me if I'm wrong).
I guess that better option would be to test the goldfingers against corresponding pins on the chips/chip (which one?) to see if there is continuity but without schematics it would be very time consuming...
Any other suggestions on how to test it (again - I have working CT1600 for possible comparison).

On the bright side - we managed to fix his GUS today (midi is not working, but I guess it's more of a software issue) - card wasn't detected in the system, but after burning GAL with code ripped from another GUS - it started working!