You can either test for voltage or current, but either will tell you whether or not you have a problem.
You'll need a multimeter and a set of probes. Set the meter for either AC Volts or AC Current, then plug one probe into the GND hole on your outlet box, and dip the other probe into your tank (if you have metallic electrical boxes, you can simply touch the screw holding the cover plate on if you poke thru the paint on the screw). You may need to adjust the range dial on the meter to get a stable reading if your meter doesn't "autorange". A voltage reading will tell you that the water is at a different potential than the house GND, and therefore, current will flow between the two. A current reading will actually tell you the amount of current flowing between the tank and house GND.