How to test a factory tach??

The easiest way would be to "jerry rig" it up on a car with a tach you kinda trust and compare it in a few RPM settings. With the service manuals posted on here, you should be able to figure how to hook it up. Should pretty much just need power, ground, and a tach triger. If you have something like a stock ignition, coil neg. If you have MSD/ other special stuff, you'll have to go to their websites. I'm running a GM HEI module, and my little Autometer triggers fine on coil neg.