Is it obligatory to excommunicate a priest who claims knowledge of the unseen and makes it his profession, and if so, am I obligated to investigate his circumstances?

An individual is not declared an infidel except after the fulfillment of conditions and the absence of impediments, such as the establishment of proof (iqamat al-hujjah). This judgment belongs to the legitimate judiciary, not to ordinary people. Soothsaying and claiming knowledge of the unseen are acts of disbelief that expel one from the fold of Islam, according to the majority of scholars, due to the Prophet's (peace be upon him) saying: "Whoever comes to a soothsayer or a diviner and believes what he says, has disbelieved in what was revealed to Muhammad." A Muslim believes in the disbelief of soothsayers in general, but it is not necessary to believe in the disbelief of a specific individual before a ruling from the legitimate judiciary. It is obligatory to advise anyone accused of soothsaying and to warn people about him. As for the one suffering from obsessive doubts (al-musawis), they must avoid delving into declaring others infidels.