How do you test a tach?

I have two tach's and am uncertain if either work. One is out of a 68 and the other a 73. Is there an easy test to check operation with the tach out of the car?

[This message has been edited by Tim Polzin (edited 04-08-99).]

You would need a signal generator to do it on the bench, or you could take an old distributor and spin it up to a know speed with a motor and run 12v through the points to a coil to simulate the ignition in a car? I had thought of doing something like this as I made a shift-light and wanted a way to calibrate it with out having having to rev the engine to 7000 in the driveway! I gave in and took it to a speedo/tach repair shop and they charged me $25, took 15 min. A signal generator is beyond my budget....Or finally you could hook them to the coil like a test-tach with some leads, 0$ probably?