There is no way of knowing until it has been fulfilled. Addionally, they must have a history of never being wrong and pass the 100% test. Don't know of anyone. As far as scripture, we know that all but one prophecy has been fulfilled. Blessings.
Some people mistakenly "believe" that prophecy is nothing more than "the strong urge to say something" - and then as you suggest in many cases they find out later that such statements were false/not accurate etc.
So then why does the Bible have such harsh penalties for lying as a prophet?
It is because the popular supposition about prophecy is NOT how prophecy works according to the Bible.
Numbers 12:
Then Miriam and Aaron spoke against Moses because of the Cushite woman whom he had married (for he had married a Cushite woman); 2 and they said, “Is it a fact that the Lord has spoken only through Moses? Has He not spoken through us as well?” And the Lord heard this. 3 (Now the man Moses was very humble, more than any person who was on the face of the earth.) 4 And the Lord suddenly said to Moses and to Aaron and Miriam, “You three go out to the tent of meeting.” So the three of them went out. 5 Then the Lord came down in a pillar of cloud and stood at the entrance of the tent; and He called Aaron and Miriam. When they had both come forward, 6 He said,
“Now hear My words:
If there is a prophet among you,
I, the Lord, will make Myself known to him in a vision.
I will speak with him in a dream.
7 It is not this way for My servant Moses;
He is faithful in all My household;
8 With him I speak mouth to mouth, (face to face)
That is, openly, and not using mysterious language,
And he beholds the form of the Lord.
So why were you not afraid
To speak against My servant, against Moses?”
God contrasts the REAL mechanism of prophecy with the even MORE explicit communication method He used with Moses.
Sadly what passes today for "prophecy" falls far short of the REAL mechanism for prophecy described in Numbers 12
New Testament prophecies are far different. In the OT the spirit came upon them. In the NT you have a born again human spirit sealed by the Holy Spirit. If the Holy Spirit wills, and you are in faith (and in order) you can give a prophetic word. Meanwhile the other prophets are to judge.
1 Corinthians 14:29-32 (ESV)
29 Let two or three prophets speak, and let the others weigh what is said.
30 If a revelation is made to another sitting there, let the first be silent.
31 For you can all prophesy one by one, so that all may learn and all be encouraged,
32 and the spirits of prophets are subject to prophets.
This shows the standard of accuracy is not 100% anymore. Because the word has to be judged. David Pawson taught too that one prophesying might start out in the Spirit, and then add something by the flesh. Since everyone has the Holy Spirit, we can discern on our own and the others can correct if they feel it is needed. The prophecies of today do not lead a nation into battle or divert from a famine. In the OT if the prophets were wrong, thousands could die. There were few in the OT but in the NT there should be several in each church as the "spirits of prophets are subject to the prophets."
Now I will answer the why? The Spirit is poured out on all flesh. God's design in the NT is to empower all believers. Many are lazy and look to a few, but the reality is that every church could have gifts flowing if they would believe God and learn how to flow.
Why did God say just two or three prophesies? I heard Bob Yandian (fantastic current teacher and author) basically say that a church could prophesy for the whole service. But that is not orderly. God desires teaching and other ministry, praise and worship etc for most services. So capping the service at two or three is exactly what Paul said.
I will say too that there is alot of prophecy today that is garbage. Some have doctrinal issues, some are in it for the money or recognition. Most of this is outside the church. Some are correct but perhaps not every time.
If you hear a prophecy, what is God saying to you? Does it contradict the word of God (clearly false) Does It bears witness? Does it give you a check or eerie feeling (Probably wrong) or it leaves a question mark. In the latter you put it on the shelf. I had a word given to me 40 years ago. I wrote it down, saw my notes just a few years back and it finally it now makes sense and is accurate.
If a church is into entertainment, it leaves no room for the Spirit to come on anybody and is too strict in exactly how each service unfolds. The cure is to replace the fleshly and really invite the Hoy Spirit to the service. The next wave will show this to be true. Most have never seen a service where the glory of God drops so much that everyone there is participating in receiving and giving from God. May God freely move in and through us all.