take it all apart and flick the needle with your finger to see that it isn't caught on anything.
basically you need to remove 3 screws from the bottom of the steering column and 2 from the piece of plastic at the top of the instrument cluster, then slowly and carefully wiggle it all out from above the steering wheel.
I have only ever bothered with aftermarket tachos so please forgive my ignorance.
Is there any way of stabbing the positive wire that normally connects to the coil onto the battery to see if the needle jumps? This is usually my quick test for fuel and temperature gauges, surely it would work for a tacho considering it's measuring the speed of *bursts* of current? haha, me being useless yet again