Yeah GutZ, check out that Scotty site, they have lots of good info there. Basically you just need an accurate voltmeter (to the .01V) and you touch your negative lead to your negative battery terminal and your positive lead to your downrigger wire. Make sure you're away from other boats, don't do it in a marina, etc., and don't do it in freshwater or near a river mouth, the salinity of the water affects the reading. Other than that, it's pretty simple, unless you have an off reading (way too high or too low), which means you have to track down the source of the problem, ie., bad bonding, grounding, wiring, etc... which can get complicated.
And rcl187, as far as not catching fish on the downriggers and catching fish on the weighted line, let me guess, are those Cannon downriggers with the "Auto-stop" feature when the ball gets to the surface? If so, that's your problem. Those particular downriggers put a charge on the downrigger cable, and sense when the ball has broken the surface of the water... some sort of ground fault circuit. What this means is, every time you raise those downriggers, you are putting over 7.0V+ on the downrigger cable, TOTALLY pushing fish away from the cables... salmon don't like voltages much over 0.8V+, never mind more than SEVEN times that.
I just talked to a guy a week ago, he told me on his last boat, he would catch 80% of his fish on one downrigger, not the other... at first I thought he meant coho, and that could be attributed to which side the kicker motor was on, but he was talking about blackmouth, fishing 120 feet down... so I asked him what kind of downriggers he had at the time, and he said, "One Scotty, one Cannon"... and the Scotty caught 80% of the fish, because the Cannon had the 7.0V+ auto-stop feature. He has two Scottys now and catches fish equally on both. Just and interesting thought...
-N.