When I turn on my finder I think I see the fish mooning me
Anyway, on a good finder you should be able to make out the rocks on bottom and useing a big jig you should be able to see it.
When testing a friends who was 100% sure his was working we parked the boats side by side and turned mine on got the reading of depth, and you could see the jig.. Turned mine off then his on, could not see the jig and depth was off by a wide margin. little checking and adjusting he got the same thing.