Lie detection may start with watching for signs of lying in a person. But sometimes you have to do more than just listen and watch. Sometimes you have to trap a liar. Here is one of the many ways to do that.
Lie Detection - Introduce An Assumption
This is a common trapping technique. Here is the essence of it: When you suspect a person of lying about some activity, you make a statement that assumes some possible fact or facts, so that the person's confirmation of these facts provides evidence of a lie.
For example, suppose that when you ask, your boyfriend says he was at home the previous night. You suspect that he was actually at his favorite bar. You ask and he claims he didn't go out. Knowing which streets he would have to drive on to get there, you say, "But I drove right by you on Second Avenue last night." In other words, you just assume that he was on that street.
If he was out, and he drove that way, he'll probably quickly make an excuse, like "Oh yeah, I did go out to get some groceries." If he was telling the truth all along, he'll just assume you are mistaken, and tell you so. If he guesses that you are unsure, he may just lie again, in which case you might try another lie detecting technique.
The idea here is to think of any facts that fit the scenario which you suspect is the truth. For example, if you think your child snuck out of the house in the night and she denies it, you could say, "But I looked and you weren't in bed last night." If she was in bed, she will say so, and you can cover yourself by saying something like, "I guess you were under the blankets."
Suppose you think an employee is lying about being home sick the previous day. Using this lie detection technique, you might say you drove by his house and noticed his car was gone. If he was lying, and he takes your bait, he'll likely say something like, "Oh. I had to go out for medicine" - a pretty good indication that he was lying (unless it just happens to be true coincidentally). Otherwise, he'll insist that his car was there, and you can say you must have been looking at the wrong house.
You always have to gauge the person's response. An obvious or fumbling attempt to find an excuse obviously indicates a lie. A long pause also likely indicates that the person is trying to find a plausible story to fit the facts which you "obviously" know.
If your "facts" or assumptions are wrong, most people will quickly say so. Of course in that case, it is easy to say that it was a simple mistake on your part. That is one of the advantages of this lie detection technique - you don't risk much by trying it.