Detecting a lie can be difficult as there are some good liars out there; but if you want to know whether someone is telling the truth, there are some clues you may find useful. However, remember that there is no sure fire way of telling if someone is lying. You have to use a wide range of observations, and in some cases, surveillance to get an idea about someone's truthfulness. So take verbal and physical observations into consideration along with surveillance before you come to any conclusions.
Look at the person's face when they are speaking to you. People who are lying often tend not to be able to look you in the eye. They also may blink a lot.
Examine their physical expressions. They may tend to fidget a lot. Look to see if the person touches their face, throat, mouth or scratches their nose or behind their ears. If they do these things chances are they are lying.
Watch the timing and duration of emotional gestures. If a person is lying, the timing is likely to be off; displays of emotion are delayed, endure longer than usual and also stop suddenly. The timing between gestures and words also will tend to be off. For example, someone who is not telling to truth about being happy with a gift might say "I love it" when receiving it and then smile afterward.
See if gestures and expressions match what is being said. A person who is lying is likely to say something like "I love you" while frowning.
Examine the person's mouth. If a person is lying and faking emotions, it is likely that their expressions will be limited to mouth movements.
See if the person places objects between themselves and you; this is something liars often unconsciously do.
Observer how a person answer your questions. A liar often uses your words to form their answer; for example, in answer to the question "Did you break the vase?" The person might answer "No, I did not break the vase." Also, listen to hear if they use a contraction, which is more likely to be truthful; in other words, a truthful statement would likely be "I didn't break it."
Listen for stumbling speech with lots of '"ums" and "aahs." People who don't really mean what they are saying often make speech errors and fall over their words.
Change the subject suddenly. Someone who is not telling the truth is likely to be happy to switch subjects quickly to avoid the situation, whereas someone telling the truth will want to go back to what you were originally talking about.