Is resorting to shaykhs to uncover a thief by seeking the aid of jinn unlawful, and what is the expiation for that, and what is the legitimate (Shar'i) way to uncover the identity of a thief?

It is not permissible for a Muslim to go to those who claim to reveal the thief, for they are considered soothsayers and diviners, whom it is forbidden to consult or believe. This is based on the Prophet's (peace be upon him) saying: "He is not one of us who practices tatar (taking omens) or has it practiced for him, or practices kahana (soothsaying) or has it practiced for him, or practices sihr (magic) or has it practiced for him." And due to his (peace be upon him) saying: "Whoever goes to a fortune-teller (arraf) and asks him about something, Allah will not accept his prayer for forty nights." An arraf (fortune-teller) is one who claims knowledge of the location of stolen items or lost things. The meaning of "non-acceptance of prayer" is that there is no reward for him in it. It is obligatory to stay away from that and not to accuse anyone without clear evidence.