Prophecy is a controversial topic but believers in prophecy use it to understand past, present and future events. Judaism, Christianity and Islam all have prophets as central figures in faith and belief.
The phenomenon of prophecy is not confined to ancient times. The Mormon Church still claims prophets are active today, and specify that the current church president is a prophet although all Christians would dispute this claim.
Many modern cults feature prophets who demand obedience and craft the cult around themselves.
All potential prophets can be classed into four groups:
The Bible cites many examples of occasional prophets - essentially normal people who have a single or at most occasional manifestation of the prophetic gift. In the Bible some are named while others are only alluded to. In each case of a true prophet's statements can be evaluated against established truth and the statements judged as true or not. Occasional prophets do not claim to be prophets; rather they are recognized as exercising the prophetic gift for a time. Outside of clear revelation, everything else the occasional prophet says or writes should not be considered inspired (although it may be true).
Self Proclaimed Prophets
Some people will couch their own ideas as prophecy to lend supposed legitimacy to their ideas. These people will tell others that they are prophets and the most convincing will attract followers. Comparing current revelations to earlier revelations will usually reveal inconsistencies in the message because the self proclaimed prophet literally makes it up as they go along to suit their changing objectives and circumstances.
True Prophets Inspired by God
God exercises very close control over his chosen true prophets ensuring that true prophets only speak truth. Everything the true prophet says and writes must be correct of the prophet is really true. Just as a deep well full of clean water becomes deadly with just a few drops of poison, a little error makes the prophet unreliable.
Satan always mixes truth and error to deceive so God can not allow his true prophets to fall into that trap. God never changes, so a true prophet must agree 100% with previous inspiration and with their own previous statements. Any contradiction of the Bible (written mostly by prophets) is enough to reject a prophet as not being from God at all.
False Prophets Inspired by Satan
If there is not 100% accuracy and 100% agreement with accepted inspired prophets, the prophet is either from Satan or self proclaimed. A prophet who promotes error is just following Satan's lead in mixing truth with error.
Note that this 100% accuracy standard should not be applied to non-inspired writer and speakers. We are all human subject to misunderstandings and misstatements. The 100% correct standard must only be applied to those who claim the prophetic gift.
Before accepting a prophet be sure and test that prophet against both the 100% accurate and 100% in agreement with other inspired work standards.