How to test / check a Tachometer? 60hp Mariner

[rcl700]

I recently replaced the switch box on and this solved a bunch of problems I was having. Im now up and running. But now I have another issue. The tachometer is no longer working. The ring light works but I have no RPM reading. The back of the tach. has three wires, black, purple, and gray in the center. I'd like to know if the gauge is bad or if there is another issue. Is this something that can be checked with a volt meter?
Please help..

Id also like to test the charging system. What should I see on the battery when the engine is idling?

Thanks