There was a tiny article / amusing note in the Reader's Digest a long time ago. A host was offended that the guest had brought a radiator with them - it was in

There was a tiny article / amusing note in the Reader's Digest a long time ago. A host was offended that the guest had brought a radiator with them - it was in the front passenger's seat in clear sight. The guest explained that it was the only way they knew to make sure that it was off. So, a somewhat old solution. But perhaps not so well-known.