I use a Humminbird Fishingbuddy with the "Sidefinder" technology - and as long as the ice isn't more than ~10-12" deep I spin it around the hole to check for fish. It works pretty well. Obviously this won't tell me how deep they are, but it has given me an idea as to distance and from there I know where to drill more holes.
If I had to guess - I'd say it's been 50% accurate/productive for me. Which is a whole lot better than blind drilling any day.