Jamieson, Fausset and Brown on Jeremiah 23:28

28. God answers the objection which might be stated, "What, then, must we do, when lies are spoken as truths, and prophets oppose prophets?" Do the same as when wheat is mixed with chaff: do not reject the wheat because of the chaff mixed with it, but discriminate between the false and the true revelations. The test is adherence to, or forgetfulness of, Me and My law (Jr 23:27).

that hath a dream--that pretends to have a divine communication by dream, let him tell it "faithfully," that it may be compared with "my word" (2Co 4:2). The result will be the former (both the prophets and their fictions) will soon be seen to be chaff; the latter (the true prophets and the word of God in their mouth) wheat (Psa 1:4; Ho 13:3).