After learning the techniques, it can become extremely easy to tell if a person is lying by taking notice of their unconscious behaviour and communication. Everybody regularly gives away small noticeable clues about whether they are telling the truth or not and it is true that the more people try not to give themselves away, the more they will give themselves away. To effectively tell if somebody is lying you need to acknowledge and remain aware of both their verbal (spoken) unconscious communication and their non-verbal (body language) communication.
In NLP (neuro linguistic programming) you are taught general eye accessing cues which can give away subtle clues, though it is not completely reliable. It is a myth that people break or avoid eye contact when lying, often liars will maintain eye contact all the time that they are lying. This states that if a person looks to the left or to the left and up after being questioned then they are visually remembering something which has really happened. If they look to the right or to the right and up then they are visually constructing a new image in their mind. If they look directly to the left then this could indicate that they are remembering real sounds. If they look down and to the right then they are remembering something which creates some kind of emotional feeling within them. If they look down and to the left then they are listening to their interior dialogue, that is the voice of their own subconscious. These cues could be the oppositte for left handed people though they should never be relied on totally.
All sorts of clues are also given away by unconscious spoken language. For example, if you are in conversation and somebody says 'yes, i see what you mean' the word 'see' indicates that they are visualising in their imagination what you are talking about. Lies will also generally be spoken in a slightly different tone to truthful comments, maybe slower and quieter or maybe pronounced louder and spoken faster. To be able to tell you first have to work out a basis for the person in question by working out what tones, pronunciations, speed and inflection they use in their voice when talking normally and truthful. You will then be able to decipher more easily when you believe they may be lying.
You also have to work out a basis for a persons normal body language behaviour before you can start deciphering whether or not you think they are lying. General and typical body language indications are scratching or touching their nose while lying, scratching or touching an ear, maintaining significant less eye contact (or opposite), becoming more defensive by folding arms, covering their mouth with their hand and of course, eye accessing cues. This sort of behaviour needs to be looked out for in clusters and be compared with the persons normally body language mannerisms in order to determind correctly whether they are lying. All these techniques are basically rules of thumb and should not be completely relied upon. It is not an easy thing to achieve and the techniques need to be practised regularly if you are going to train yourself to know when somebody is lying.