Here's an example of how you would do it, but may not apply to your scenario: You can use a multimeter to test for continuity between two wires that should not have continuity, if they do they must be shorted by touching.

Say you know the short is blowing a fuse on a certian circuit, you can unplug the harness from a certain point, and disconnect the battery. If you know by reading the wire diagrams, that two wires are entirely seperate circuits, and they have continuity (a zero reading of resistance) than you know they are the problem.

Set the MM to resistance, the ohms symbol. When current can flow from the red to the black on the test probes, there is continuity and you will get a reading. The default would be infinite resistance.

if the dealer is right, you are going to have to figure out the construction of the evap emissions systems and its wiring.

chris