How can I trace the wires of a circuit?
one device would be to use a multimeter. They have something called a continuity tester, whereby you take the two leads. the one end gets pressed against the one end of the wire, and the other end goes to the other end. if there is a connection, then the multimeter will either show on screen of buzz. ... the real problem is to run a temp wire all the way from  the one end of your wall switch to the other end being tested. (that is just to effectively extend the length of your test leads of the multimeter.