There are stud finders and then there is the Milwaukee Sub-Scanner M12. Oh, it can find studs alright—but it can also find the location and depth of rebar, PVC, wood, PEX, metal and live wires under 6" of concrete.
Actually, it can tell you the exact position and composition of just about anything under concrete, gypsum, ceramic tile and marble. Of course, a tool like this is designed for professionals—and it has a price tag to match. Expect to pay $310-$350 in most places. [Milwaukee via Toolmonger]