Tach Test on 1997 225hp OceanRunner

I have a 1997 225 oceanrunner that has been converted to the old style big red plug on the wiring harness. My tach was working fine on the last trip out last year and then started bouncing all over before giving up the ghost. It hasn't worked at all this year in four trips out and approximately 10 hrs running on the engine. I have been nowhere near wot to be concerned about the revs, but I understand that this could be a charging issue, although the battery is still fully charged, as opposed to a bad tach. Is there a simple test with a volt/ohm meter to determine which problem I have?

Thanks in advance.