Hello igeorge,
Could you elaborate a bit on the conditions you want to test the device? It seems you want / need to test during the welding operation. Can you install a tiny reed switch along the power wires? Honestly I do not know if a reed will activate on an A/C load wire, a coil like a Tape Head or a guitar pickup will. If you do not need to test during operation, then a simple continuity test should work, neon lights used to work very well for that.