The paper examines the "Antiplagiat" ("Anti-plagiarism") system, describes the characteristics of alternative online plagiarism checkers: Etxt Antiplagiat, Text.Ru, AdvegoPlagiatus, Content-Watch.Ru, DCFinder. It defines the problems of detecting hidden artificial increase of text uniqueness by "shadowy systems". It proposes an algorithm of preparing the text for plagiarism check. It elaborates the main concept of a new service, which allows to check Cyrillic texts for borrowings from English-language sources.