Thou shalt come unto the priests, the Levites,.... The priests that are of the tribe of Levi, as the Targum of Jonathan, and so Jarchi; for Aben Ezra says there are priests that are not of the genealogy of Levi; such there were indeed in Jeroboam's time, 1Ki 12:31.
Maimonides observes, that it is ordered that there should be in the great sanhedrim priests and Levites, as it is said: "and thou shalt come unto the priests, and the judge that shall be in those days, and inquire"; judge is here put for judges, of which the great court consisted, being priests, Levites, and Israelites;See Gill on "De 16:18", though others think that only a single person is meant, such as Othniel, Ehud, Gideon, Samson, &c.
but then as there was not always such an one in being, I should rather think that the judge here, if a single person, is the president or prince of the great sanhedrim, who succeeded Moses, and sat in his place; and of him and his court, the priests, and Levites and Israelites that composed it, inquiry was to be made:and they shall show thee the sentence of judgment; give their judgment in the difficult case proposed, and declare what is right to be done, and what sentence is to be pronounced.
Hilchot Sanhedrin, c. 2. sect.