How do I detect and repair faulty wiring?
Use your non contact tester, and start tracing the circuit back to the panel.  Check each device along the way, and keep a close eye out for GFCI receptacles that have been tripped.

You may want to pick up a multimeter, and test for voltage at the switch, and at the fixture. Water could have entered the fixture housing, and fried the motion sensor. So make sure you check the wires feeding the fixture.

If you don't feel comfortable working on live lines, call in a licensed electrician to have a look.