I verified mine by putting a 6800 rpm pill in my 6AL and hitting it. My autometer shows 6800 rpm at the rev limiter. Plus it's just one wire from the MSD to the tach.
A friend of mine FREAKED OUT when I drove her Z and wound it up to ~7K. Apparently her tach was showing 8500 or something. I was unable to convince her that her tach was wrong, until I asked why her valves hadn't floated if it was really at 8500 rpm (stock springs, mild cam). She still hasn't fixed it.
I've never had good luck with ANY of the stock gauges. Tachs are innacurate, speedos jump, temp gauges are CRAP, oil pressure gauges are a just so vague that they're useless, I've been thru several ammeters, and the gas gauge looses 1/2 tank in the first 1/4 tank but I think that is the shape of the tank more than anything else. I've only seen one working clock ever, if you want to count that as a gauge.
I agree with Lockjaw, just replace if you want to really know what's going on.
Jon