Jesus said that they would be 'known by their fruit', and, as far as I know, he preached a very biblical message, even against the 'prosperity-gospel'.
For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
I believe David Wilkerson's book was, "The Cross and the Switchblade".
I know next to nothing about modern day prophets but would gladly receive Biblical teaching on this subject. I was going to a church where messages
were given in tongues and then interpreted as prophecy. I was told that whatever the person interpreted was from the Lord. I'm inclined to be skeptical until I learn something from the Bible on this.
I have listened to many of Wilkerson's sermons. He was truly a great man of God. He had 2 main themes to his preaching.....Pray and read the Bible....that is it. All you need to know can be learned by doing those two things, diligently. I have never heard Pastor Wilkerson refer to himself as a prophet, but many times said he was a watchman. I love his sermon where he tells of what brought him to New York, prayer led to obedience, even though he did not know his purpose until he obeyed. A lot of his sermons are on sermonindex.com.