Factory tach 1968 GS

I have a tachometer that I believe fits an early 65-68 Buick with the Consolette. It has 6412531 printed on the back. How do I test it?

 

The case is grounded to the chassis. The blue wire to negative post on coil, white wire is light.
Test on any 12v coil ignition.

 

Thanks. I was hoping to bench test it with a multimeter.

 

You will have a hekkin' time trying to create the collapsing field of a coil in rapid succession that the tac transformer samples to energize the field windings that swing the needle.

Or more simply, I don't think you can do it without a pretty sophisticated bench setup.

That does not mean someone hasn't done this because they were told it was a lot of work.

 

Thank you.