Jamieson, Fausset and Brown on 1 Corinthians 14:29

29. two or three--at one meeting (he does not add "at the most," as in 1Co 14:27, lest he should seem to "quench prophesyings," the most edifying of gifts), and these "one by one," in turn (1Co 14:27, "by course," and 1Co 14:31). Paul gives here similar rules to the prophets, as previously to those speaking in unknown tongues.

judge--by their power of "discerning spirits" (1Co 12:10), whether the person prophesying was really speaking under the influence of the Spirit (compare 1Co 12:3; 1Jo 4:13).