Even better, I learned as a newspaperman, is to quote them word for word in a lengthy passage, so they realize they were taped without their knowledge.
That gots particularly funny one time when one denied having given me a statement the tape of which I subsequently played for my editor, who backed me all the way once he heard the liars's statement for himself.
-archy-/-
Best way to do that is to start taping him, let him know it, let him take the recorder, and have a second one going the whole time.