Forensic psychology expert opinion is an evidence in a court, especially in criminal cases. In general opinion the psychologist has to make a statement about the behaviour of the investigated person/child, referring to his/her general reliability. It is like a liespotting process, verifying the statement by the results of valid psychodiagnostic tests in conformity with a deep anamnesis and the observation of the subject. The expert’s examination has a strict protocol, that needs high-educated and well-experienced psychologist. Lye has no universal signs, moreover, there are psychiatric illnesses with intensive imagination, handling the unreality as a reality, like children use to do. For giving a short review about the complexity of this professional work, the topic is presented by the discipline of children.