Yep, inductive tracer, and a fish tape...
Recently traced out my terre cotta drains that way with an old fish tape. (Drain snake broke and needed to know where to dig for the plumber.) Hosed it off as it came back out, but I'm still weary of it... Man, I'm glad I'm not a plumber.
Anyway, an underground inductive tracer, many rental joints have them. Or any circuit tracer w/ a ground coil, and some power. Your phone toner wont cut it...
Pushing a fish in it might be risky, could come up in a panel. Have you given thought that it might not go anywhere?
