The main aim of the following paper is to draw the attention to problems concerning control of informants and assessment of the credibility of information provided by informants. So far, this issue has not been analysed thoroughly in Poland. Still, it becomes very significant when the information delivered information refers for example to preparations for terrorist attacks. The various operational methods which have been used so far to verify information are time-consuming and do not dispel all doubts. Objections are also raised about using polygraph examination since informants are often opposed to such formalized cooperation. In those situations, the focus should be on an alternative method of credibility assessment, i.e. the voice analyser. According to its producer, this is a better choice than the polygraph because it is not subject to various limitations. However, there are few methodologically appropriate scientific studies concerning the diagnostic value of voice analysing examination and those existing are mainly of an experimental character. Yet, devices of this kind are used in many countries of the world, both by security services and civil institutions. Several insurance companies in Poland make use of them and an interest in their functioning is shown by secret services and the police. Before these devices are used on a large scale in operation and investigation, two issues should be established after comprehensive scientific research; these are the actual accuracy in relation to the Polish population and an adequate scientific methodology.