There is described the methodological approach to the linguistic diagnostics of message reliability in the course of a psychological and linguistic expertise. The urgency is determined by the need to develop the valid methodology aimed at establishment of speaker’s attitude to the information transmitted not only from the position of truth/falsity of a message but also from the point of fullness and character of speaker’s knowledge about the information given (epistemic purpose).

There is described the methodical approach to finding out the derivative texts produced by rewriting and automatic texts production aimed to popularize the potentially extremist speech works. The scientific novelty is determined by the fact that the issue of variable statements of one and the same referent situation was never raised in court linguistic expertise of extremist materials.