The ones who are to judge whether a prophecy is God-given are the leaders in that particular assembly where the prophecy is uttered. The prophecies should not be spoken without the leaders commenting upon them. The basis of their judgment is a supernaturally given discernment by God.
The Bible says that prophetic utterances should be allowed in the churches. “Do not despise prophecies” (1 Thessalonians 5:20).
Prophecies should not be despised, but they should be tested. “Test all things; hold fast what is good” (1 Thessalonians 5:21).