Most people use a digital multimeter with the measurement scale set to AC Volts. One lead in the water, the other into an appropriate ground, commonly the grounding prong in an electrical receptacle.
As for what amount is considered bad, that's a bit of a tricky one to answer, but I'd start investigating the electrical components I have submerged in the water if you start seeing anything above the range of 20 to 30 VAC. As I mentioned in my earlier post, certain fluorescent lighting fixtures may induce a fair amount of voltage into the water, especially if they're close to the surface. That goes for MH lights as well depending on type of ballast and where it is located. Also worth mentioning that it's a good idea to ground individual reflectors on your T5HOs if that's feasible.
Here's some further reading which may clarify or be of use:
http://angel-strike.com/aquarium/GFI...alDetails.html
http://angel-strike.com/aquarium/GroundingProbes.html