You can check the sender with an ohm meter . . . I can't find my Bentley manual, so I have no idea what the reading should be, but it should be low ohms when the float is raised and get progressively more resistance as the float drops.

My experiance has been that the #1 cause of the gauge not working is bad wiring . . . usually just bad connections.  Make sure the sender is grounded!  The little device on the back of the gauge (the "exciter") is (again, in my experiance) the second leading cause of the gauge not working.  Next would be the sender and then the gauge.

But, keep in mind that I'm far from an expert! The truth is I always just swap parts around until I find a combination that works (you can do that when you have a dozen or so parts cars in the back yard) . . . if I get frustrated before I find that combination I usually just go without a gauge for a couple of years, then I'll try to fix it again.  That makes driving more of an adventure anyway.

I think an easier count to make would be to take a poll of those with WORKING fuel gauges in thier A/C VW's.  My Fastback is the first one I've had that it seems to reliably work (now I've done it....).  I must have fixed the one in my Ghia a few times before giving up on it, and I'm pretty sure the Keedy Ranch up there doesn't have a working gas gauge present.

Seriously.  The Bentley Manual has a very nice description of how to test the gauge.  I'll try to sum up if you don't have one.

fuel guage that never moves from full has a grounded control circuit.  Disconnect the control wire from sending unit, if guage falls trouble is in sending unit, if not control wire is grounded somewhere between sending unit and gauge.

if guage doesn't register at all, remove guage wire from sending unit and ground it, if guage goes to full sending unit is bad or not properly grounded, if guage still doesn't move wire or guage is faulty.

Hope this helps

VWTrav