The simplest is known as the knock test.... Telegraph poles are still sometimes tested by hitting them with a hammer, a rotten pole results in a dull thud, like hitting the bark on an oak tree, a solid one sort of rings like when you drop timber on a hard surface.... On a more technical level you can get poles tested by ultrasound. This requires the use of specialist surveying equipment such as a resistograph, or an Ultrasound Picus Tomograph machine..

Thank you Andy & Frank, very useful leads. I now have a utility policy tome showing how well advanced the art of pole watching is. Would not have come up with this without your input.