Behavioral analysis has developed tools that can increase the likelihood of detecting lies when you are talking with people through body language, speech patterns, etcetera. A fascinating study, as any internet search on "lie detecting" can show.

For fun, study up on lie detecting a bit and then watch any political debate!

Have the kids study up a bit and then take them used car shopping! Warning: the kids need to understand lie detecting is an art that is not infallible, requires practice, and is often of best use if you do not confront the suspected pervaricator [do not show your hand].