Yaknow that's a good question. The only way I know for sure is if I'm using a downrigger, otherwise it's a bit of a guess. I suppose if you're seeing fish on the fishfinder at a certian depth, and you let out line trolling untill you start catching those fish, then you know. But the amount of line out and the trolling speed must be the same to repeat that depth. Also the lure type and/or weight must be the same. I'm sure there's somebody out there that's formulated the recipe for all the variants of speed/line length/lure weight to know the depth you're trolling at. Just can't remember seeing any graphs or tables about it before.
I have a jig with a face like this!:eek: