Before we can understand if a prophecy is from God, we have to answer the question-what exactly is a prophecy?
A prophecy is something God shows his spokesperson or prophet (Numbers 12:6.) We can’t understand prophecy without realizing that God uses man to speak to his people corporately (Exodus 20:19.) Now the prophecy is not created by the man, it is not brought about by the imagination of the person (2 Peter 1:20-21.) The prophet is simply speaking and describing what God has shown.
So if we understand prophecy is a genuine God-ordained method of speaking to man, how do we deal with the scriptures that we must not add or take away from scripture already revealed? In Deuteronomy 12:32 the Lord clearly states that YOU shall not add or take away from the law, yet in later books God continue to add to the revelation of scripture. This command continues on through to Revelations (Rev 22:19) where we are commanded continually not to add or take away from what He’s told us. The distinction is that man cannot do this but when God reveals something to His people, HE by his own determination must use a prophet (Deut 4:2.)
So in short: a prophecy is anything that is from God, communicated by His prophet, to His people.
An additional question to consider is: what are the substitutes organized religion has for prophecy or forth-telling of the Word of God. Church Councils acting as spokesmen for God have declared creeds and particular doctrines to represent their body of churches as ‘orthodox’. This is the height of false prophecy, to stand as the pharisees did in the seat of Moses’ yet having no God-given authority to back up their decisions. The only actual authority they have is derived from the political nature set in place by a system of man’s governance rather than the Holy Spirit through God ordained men.