If you have a light fixture that is not attached to an outlet, say for example a downlight, you may be able to use a neat little adapter called a keyless socket adapter. It works by simply being screwed into the edison base of a light fixture which is then converted to a non-grounded outlet plug.
Simply plug the transmitter into the adapter and it functions as if it were plugged into an outlet.
Follow the same instructions as in Step 5 of this tutorial to trace this lighting circuit back to your panel.
