A diviner is a specialist in a technique for obtaining hidden knowledge, and the defining point is the technique: this is what separates the diviner from the seer, who claims a faculty.
Method, not faculty
The diviner works a system, the casting of lots, the reading of shells, bones, cards or entrails, the interpretation of dreams or omens, the drawing of a figure, and reads from its outcome the answer to a question or the hidden state of things, so the authority rests on the mastery of a method rather than on an inward gift. This is the clean distinction from the seer: the seer sees directly, the diviner reads a result produced by a technique. The diviner may serve to reveal the future, to find the cause of an affliction, to locate what is lost, or to determine the will of a god.
A universal office
Divination is one of the most widespread of all religious practices, and the diviner appears across virtually every tradition, from the Ifa priest of the Yoruba reading the palm nuts to the reader of the Chinese classic of changes, from the ancient inspector of entrails to the modern reader of cards, and the techniques are enormously varied while the office is constant. Entry is by training in the particular system, often long and demanding, and the diviner standing rests on skill in it and on the record of its results. The office is the practical, method-based counterpart to the seer, and its ubiquity reflects the universal wish to know what is hidden and to have a reliable procedure for finding it out.