Look to the other party. Of course, in someone else's soul will not drop, people's thoughts will not read. Besides, many criminals tend exceptional acting skills. They are well able to pretend. However, any more or less observant and thoughtful man is quite able with a high degree of probability to determine if he is sincere companion, or hiding something, and that was a lie.
The fact that a lot is still unavailable to the permanent control, even with undeniable artistic talent. Facial expressions, gestures, posture, tone of voice - all this can eloquently be characterized by person. Only real aces last a long special training, can perfectly portray "their", as "strangers" (how here not to recall the legendary Stirlitz, for example). And the usual liars, cheaters, robbers aren't so talented, fortunately for normal people.
Therefore, talking with someone, pay attention not only to what he says but how he says. What are its gestures, what position he takes, not the voice. If your partner is stubbornly avoiding to look you in the eye, if it constantly pulls the tip of the ear or a mustache, often touching the nose or chin, smooth hair, fidgeting on the spot or, alternatively, sits still and tense, "like a broom" – it is an occasion guard.
A very good test of sincerity is as follows. If we are talking (especially on his own initiative) on something fun and you together began to laugh, watch his eyes. The fact that the circular muscles around the eyeball are reduced reflex, regardless of the wishes of the person. Therefore, if the emotions of your interlocutor is sincere, if he really funny, and the eyelids and the skin around his eyes begins to move. Otherwise, it's an act that feels fun. Here's another reason to wonder what purpose he fool you?
Of course, it is not necessary to go into extremes, all fear, seeing almost everyone a potential enemy. But reasonable vigilance and care, and yet no one never hurt.