Electric Underfloor Heating Cable Repair.

Humm not sure about the wisdom of the repaired section lacking a surrounding earthed braid, especially if you're going to be a slapdash about actually making sure the bare crimps are covered with heatshrink.

But in answer to the question, if it's the parallel type heating cable, then it should work normally up to the point of the fault, so seeing where the floor starts being cold should be a tell-tale. Infrared camera would make it easy, or I've a vague recollection of some heat sensitive sheets than can be laid on the floor that change colour according to the temperature.

Series type cable would be trickier...

   - Andy.

Had a Warmup technician come around to a job i did which had a fault under Amtico flooring. He basically put a high current through the electric mat, then got out his thermal imaging camera and searched for a hot spot (i actually found it first with the one on my phone:)). It turned out to be the factory made joint half way along the wires. Took a piece of the Amtico up, cut the flooring carefully and there it was. Managed to make good so not noticeable.