I've used the following trick successfully to trace an in-ground, empty conduit:
Since the pipe (or part of it) is metal, you cannot attach the toner unit of the tracer to your fish tape, as the tape will make ground with the pipe and negate the tracer readings, so what you do is tape a scrap roll of insulated wire to the end of the fish tape and feed both into the pipe. Once you hit the end of the run (either the break or the next box), you cut the wire and attach the toner to it, then the other side of the toner to a large screwdriver and stick it in the ground. Now trace the wire in the pipe.
It's important to make sure the wire is insulated from the fish, so it's a good idea to wrap the cut end with a little rubber tape before you attach it to the fish, just to be sure.
Follow the trace until you loose it, and that should be pretty close to the end of the run. If the readings at the end of the run seem funny, you can just paint your line as far as you can, then use the length of the wire and/or fish that come out of the pipe to give yourself a pretty accurate distance.
If you don't own an in-ground tracer, you probably want to think about picking one up if you're going to be doing this kind of work. I didn't think I needed to own one until I bought one, and it's probably one of the best investments I've made. It paid for itself after 1 or 2 jobs, and it's made and saved me quite a bit of money since.
Good luck!
