Mercury is easy to test for if you are so inclined to pay a lab to do it for you.
It accumulates in fish, especially older ones, because of the food chain and what they consume. I suspect the levels in grapes are very low to non-detectable. Not something I would lose sleep over anyway.
I agree; however, there are a number of Superfund sites in that area. I would be surprised if soils, water (surface waters, groundwater, and irrigation water), and grapes/wines were NOT being tested regularly. There was a big dust up a few years ago about arsenic in wines. I don't know what happened with that lawsuit, but I seem to recall that the levels, although detectable, were so low that one would have to drink gallons of wine to get a harmful dose (and most likely die of liver disease, rather than from arsenic poisoning).