You can test pretty much everything but the speedometer without too much difficulty.
The clock runs on 12 volts DC - so it's just a matter of hooking up 12 volts to it at low current (i.e. DO NOT hook it directly to a car battery). Odds are it doesn't work anyway
The ancillary gauges (fuel, oil, etc.) are powered by 5 VDC, so again, getting a source for say 6VDC (4AA batteries in series), you can see whether the gauges function. In my experience, these particular ones seldom break, so you should see the needle sweep as you apply voltage.
Tach involves a bit more work - ideally you have an older 8 cylinder car in running condition with a separate coil. Connect the tach as it would be in the dash - i.e a positive lead from a 12VDC accessory circuit such as the radio, and a negative lead from the -ve side of the coil. If it moves, then hey, it works. Bear in mind that the originals were notoriously inaccurate.
I believe that RT Engineering
http://www.rt-eng.com makes a 'tach exerciser' which you can use to test and calibrate the tach - might not be a bad investment
As for the speedometer, well, you're on your own. You'll know soon enough whether it works.....
I suppose you could put a spare speedo cable into a drill chuck and see what happens, but I have no idea if it will spin fast enough to allow you to test it properly
YMMV
