Multimeter and Calibrated Zinc
Thom,
There is a local guy who presents are Milltown Sailing each year who uses a calibrated zinc to a multimeter to give you an exact reading. Otherwise, you can attach a very small piece of zinc to a DVM lead, drop it in the water, and then test various points on your boat. I'll see if I can come up with the specifics since he provided handouts.
regards
shaun