If anyone could anyone could answer that question, with absolute certainty, they would be a NUCLEAR GOD!
Here's one approach that works, put the tools in their hands! If your out on a job, put the meter in their hand and have them check out a few known sources. Most will get an immediate respect for what their working with, since they found it themselves, seeing is believing! Of course it usually turns into a one hour found it game, among the crew, but it gets the point across! Meters with audible features, at high volume, work best!
PS: Be prepared to answer questions at break!
Hope this Helps, RG