The easiest way would be to test each axle end brake wires with a clamp-ammeter. Amperage will spike if there is a loose wire or shorting magnet. I've never heard of a magnet failing.
I'd strongly suggest to just rewire the whole brake system. Construct the wiring into a "Star" configuration rather than the typical daisy chain. If an owner wires each brake pair of wires to a terminal strip on the tongue of the trailer, diagnosis is a snap since the pairs are right in front and easily checked with a meter for any faults. The Star configuration will also allow the most power possible to each brake.