A character with this proficiency can perform various rituals that purport to fortell the future (though none of the rituals actually work). The rituals might include unusual devices, sonorous oratory, cards, runes, palm reading, animal entrails, and more. A good performance and some plausible deniability is more important than true accuracy.
A successful proficiency check indicates that the audience believes the predicted fortune. If it fails, the sham is discovered, and the fortune is not believed. Fast-talking modifiers may also apply at the DMs discretion. Note that a PC can never be forced to believe a prediction.
As an option, the prediction of the fortune-teller can actually come true on a result of 1 (or some other number chosen by the DM before the check is made).