It is my firm opinion that since you must manipulate others with your language, it's best for you to know what you are doing and do it with skill. This comes as a surprise to many people and contradicts popular wisdom. Our mainstream culture devalues hearing, giving far more weight to such visual information sources as the printed word, movies, television, maps, charts, and diagrams. We tend to assume, therefore, that we will be better able to tell that people are lying if we can see them. For at least two very good reasons.
People are good at putting on false expressions and can become highly skilled at doing so.
Learning to assume false intonations, on the other hand, is very difficult indeed. Most people, especially under stress, are hopelessly bad at this. You have to observe facial expression, posture, gesture, distance the speaker maintains from you, etc. When you have nothing to attend to but the voice, on the other hand, none of those distractions can interfere. When you have reason to wonder about someone's honesty, then, the best way to proceed is to talk to that person on the telephone.