Grounding probe you can buy from
www.jlaquatics.com
To test for stray current, you need a voltmeter (a $15 multimeter from Canadian Tire will do), then you put one probe into the ground of a power bar, and the other in your tank, and see if there are any volts reading on the meter. Every device (pumps, heaters, etc.) puts out some stray voltate, the key is to measure it every month or so to ensure it does not increase. The grounding probe has to be out of the tank to test for stray current. The grounding probe "grounds out" the stray voltage for you. But it does this at the cost of creating current, so too much is bad. If you have a device that is putting out more than a couple volts, I would probably replace it.