The Tach works by using the points as a source of a PWM wave. As the points open and close they approximate a square wave at a frequency corresponding to the firing of the cylinders. So if you have a signal generator you can create this wave for a known perfect RPM point. Let's say 600 RPM (because it makes the math cleaner) So at 600RPM you're at 100RPS or 100Hz. So a 100Hz Square wave fed through the Tach should create a reading of 600RPM. What I'm not sure of is the voltage, I think it should be 12V but it might be 6V, this point has always slightly confused me.
The Temperature, Oil, and Fuel gauges, all use a scale of 0-5VDC (Based on the voltage regulator on the back of the tach cluster) so you should be able to feed them a straight voltage, sweep it from 0-5VDC (make sure you current limit yourself, shouldn't take much to sweep them) and make sure you get good sweep from the needle.
The Ammeter measures current, and is a little harder to test. I'm not sure what its' "Nominal" or "Centered" value is supposed to be, someone a little more knowledgeable would need to chime in on that.
I'm not sure if you can really test the speedo without having some way to spin the cable at a known "speed" and I'm not sure exactly what speed it spins at since I know there are some gears involved in that.
Hope this helps!