If you want to measure current. You will need a ground probe, and an amp clamp. Stick the ground probe in the tank, and clip the amp clamp around the wire.
If you are looking for stray voltage. You can use a voltmeter. Put one probe in the tank, and the other to ground. Anything under 90 volts, I wouldn't worry about. It is just induced. If it is over 90 or close to 120. You have a real voltage, and something is shot in your tank.
I hope this helps...