Summary: Don't call them a prophet until you see the fulfillment and the 100% batting average.
19 Quench not the Spirit.
20 Despise not prophesyings.
21 Prove all things; hold fast that which is good.
We are not to extinguish the Spirit, which is what cessationists do by denying that He can work in all the same ways He did when He was first given to the Church. They allow the fire of God to burn in some areas, but want to control Him by using their theological extinguisher in some areas as a means of controlling the fire. Let Him be God and control the flame. You may just want a campfire when God wants to burn the house down to the foundation so He can rebuild it. You want to set up a firebreak when God wants to burn the entire field so that all the weeds and deadwood can be burned to allow a new growth to come up.
We are not to despise prophesyings. Many do that as I did for many years thinking those times were over as I was taught in seminary. By not seeing them as still valuable we despise them. There is nothing that the Church needed in the first century that the 21st century Church does not need as well, if not more so in an age of apostasy and iniquity.
Often the "prophet" was a fly by having moved on to somewhere else and cannot be called into accountability for their false prophecy. Even if you can get to them they may "spiritualize" the prophecy or find some other way to justify their error.
There is no evidence of any biblical prophet that was ever wrong or vague. Indeed, a prophet whose prophecy did not come to pass did not get to make a second mistake. Even if it came to pass but drew people from God to another God that prophet did not have a long career.
My son received an awesome prophetic word of how his ministry would be to finance the Kingdom and it was laced with verbiage clearly with an earthly application. Shortly after that he died and some want to make the interpretation a heavenly one saying that he is fulfilling the prophecy in Heaven. In essence, the prophecy was not wrong, but the initial interpretation of it was wrong.
That may be trying to make the best of a bad situation, but not accurate. It is also trying to make the prophecy accurate. No biblical prophet ever had to make his prophecy accurate by twisting the meaning. They were always accurate and came true in the smallest of details. My son's was a prosperity gospel kind of prophecy, but not a biblical one. Admittedly, this threw me and just about made me toss out the whole thing and go back to being a cessationist, but I cannot for I have heard God clearly in ways that cannot be explained any other way than supernatural.
Some cessationists might caution me that maybe what I heard was from the other side. Odd, is it not, that they believe that satan can still speak to people today, but not God? No, the things that I have heard were from God. Indeed, they are nothing the other side would want to tell me because they increased my faith in God and kept me pressing on rather than quitting in despair.
That is why verse 21 says to prove all things and hold all to that which is good. Despise not prophesying, BUT prove the prophet! God does not dispense His gifts on the trial and error method or grade His prophets on a curve.
As Dr. Michael Brown speaks to in his book, "Playing With Holy Fire", we allow too many fakes and abusers of the gifts to go unaccountable for their false prophecies and other actions. These "prophets" bring confusion and hurt to the Body and cause the enemies of God to blaspheme.
21 Not every one that saith unto me, Lord, Lord, shall enter into the kingdom of heaven; but he that doeth the will of my Father which is in heaven.
22 Many will say to me in that day, Lord, Lord, have we not prophesied in thy name? and in thy name have cast out devils? and in thy name done many wonderful works?
23 And then will I profess unto them, I never knew you: depart from me, ye that work iniquity.
We need to be discerning and not fall prey to everyone who can say the right words. Even miracles are not an automatic vetting of the person. This why doctrine is important. The Bible does not bind the Spirit, but the Spirit will not do anything contrary to the Bible. He was instrumental in writing it so He has bound Himself to it because His character and truth does not change.