The Church's Right of Exorcism

The church's official right of exorcism isn't that easy to get hold of because the us c c b will only sell it to bishops. So here's what the church tells exorcists. What criteria are exorcists suposed to use when they're diagnosing a demon in preparation for a possible exorcism? Here's what it says: "An exorcist should not proceed to celebrate an exorcism unless he has ascertained with moral certitude that the one to be exorcised is really possessed"