As
@JohnD says, proving dead is a long-winded process for items one can't unplug, and likely most of us don't follow the process to the letter. In fact only one job was I required to walk around with a proving unit, in the main I am naughty and just you two testing devices, the likelihood of both failing is low.
Often, even when isolated, I treat it as if live. But testing it is live, and then showing dead, is normally good enough. But without that live test first, so easy for meter probes to make a bad connection. I use a non-contact tester, as well as a tester with leads, so I am unlikely to be caught out with a bad connection with my leads.
This is my favoured tester
it has both non-contact voltage testing and voltage testing with leads, and can detect current flow without disconnecting the wires. Always use a neon screwdriver, second string to my bow, should I have a fault on my meter, then it will detect voltage, and also if one disconnects a neutral or earth with current flowing it will likely light. Wearing gloves can also serve as a third string, but the neon screwdriver will likely not work if wearing gloves.
