Need help with gauges PLEASE!!!!

Thanks guys,
I'm not sure what the correct type/name of the gauge cluster is but it's a V8 67 barracuda formula S, it has the speedo, vacuum/performance gauge in the centre and then the volt/temp/fuel/oil gauge.

How do I know 100% that the fuel gauge is cooked?. I clipped a wire on the back of the gauge last night (sender pin) (still in the car) and briefly touched it to earth. The gauge went all the way to full, but went up pretty fast?. Is there any test I can do in the car to check if it's OK or not. Shall I take the sender unit out of the tank and move it manually to check the gauge?. I wonder if the voltage regulator has fried and this is why the gauge moves up so quick when I tested it? I figured if it's burned out why does it still move?.

Any ideas???