There is a Sheik who can say if somebody has Sihr or not - if he knows person's name and her/his/mother's name. How is it possible, what kind of knowledge is it? Is it forbidden to use Muslim jinn for helping people?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions. If that Sheikh who tries to know whether the sick person is bewitched or not knows his (sick person's) name or his mother's name, then he is apparently a liar fortune teller against whom the Prophet (Sallallahu Alaihi wa Sallam) warned us in his saying: Whoever goes to a diviner or a fortune teller and believes in what he claims, then he will be a disbeliever in that which has been sent down to Muhammad (Sallallahu Alaihi wa Sallam). [Narrated by Ahlus-Sunan from the Hadith of Abu Hurairah , and classified as authentic by Al-Hakim ] A Muslim must recognize and beware of such fabricators and diviners who seek the help of the jinn and devils in order to consume other people's money unjustly. As for the question as to how can one know such information (about the person's or his mother's name and whether he is bewitched or not, etc.), it can be answered by the fact that such diviners get knowledge about these things regarding the sick person from the jinn and devils about whom Allah Says (interpretation of meaning): {Verily, he (Satan) and his soldiers see you from where you cannot see them.}[7:27]. There is no point in knowing how he gets such information, for it is a Haram (unlawful) thing. A Muslim must be cautious of such people and must beware of believing them. The true and straight Sheikh is the one who treats by the Holy Qur'an and Sunnah [supplications from the Tradition of the Prophet (Sallallahu Alaihi wa Sallam)] and not by asking for the sick person's name, etc. For more details, please read the Fatwa: 83703. Allah knows best.