As far as checking gauges on the bench.. temp is easy to test.. just stick the probe in a pain of hot water and see if the needle rises!
Charge meter can be checked with a large load resistor (or 6V headlight) and a 6V battery with the gauge tied in series with the load. Strap a digital meter in paralel with the original gauge to check its reading for accuracy.
Speedometer is easy.. just hook up the speedo cable to the speedometer, and chuck the other end of the cable into a cordless drill and spin it about 300-500 rpm.
Not sure how you'd test oil pressure.. short of connecting an air hose to the back of the gauge and putting a known amount of pressure into it.
