I say the two biggest factors are speed and current direction, I've found that many times if your trolling direction is off 5-10 degrees either going into or with the current is the difference between great fishing or slow fishing.Also a sub surface probe ( depth raider , fish hawk) will tell you when you are truely running with or against the current, but the simplest wat to tell is when all your gear tracks straight. You willfind many times that trolling straight into the waves or with them is not completely following the current flow. Hope this helps alittle.