Let the prophets speak two or three, and let the other judge.
Two or three - at one meeting (he does not add "at the most," as in 1 Corinthians 14:27, lest he should seem to 'quench prophesyings,' the most edifying of gifts), and these "one by one," in turns (1 Corinthians 14:27; 1 Corinthians 14:31). Paul gives similar rules to the prophets as he had to those speaking tongues.
Judge - by their power of "discerning spirits" (1 Corinthians 12:10), whether the person prophesying was really under the influence of the Spirit (cf. 1 Corinthians 12:3; 1 John 4:1-3).