One way to identify a false prophet is that they will dismiss anything said that is initially backed up by scripture references taken from what we now call the Old Testament.
What they avoid, however, is the fact that the designation of Old Testament which we traditionally ascribe to the books from Genesis to Malachi is really just a designation based on the tradition of men. The Testament itself begins around Exodus 18 and ends at Deuteronomy. The Law is the Testament. The rest is comprised of records of events before and after the establishment of that testament, along with worship poetry, and god-inspired philosophy (Ecclesiastes).
Adam, Noah, Abraham, Isaac, and Jacob, for example, did not live under the Old Testament, for example. Furthermore, in Romans 8:4 we are told that Christ and the Holy Spirit were sent to establish the righteousness of the Law in us.
However, let’s let those red letters in your Bible settle this for sure. Christ quoted Deuteronomy more than anything else. Christ himself was the mediator and Testator of the New Testament. He was the one establishing the New Covenant, yet he did not dismiss the old covenant or the writings connected to it as being invalid. Shall we then dismiss what the TESTATOR of the new Testament did not dismiss? Do we have more authority than the savior? I think not.
Part of the motivation behind this is that the Old Testament contains some passages which help to expose false prophets. Think for a moment. How many popular prophets can you name who teach on the prophetic out of Deuteronomy 18? Probably none. That passage contains a few verses that would make it hard for them to explain why they have a decades-long track record of prophecies which never came to pass.
Thus, a careful study of scripture will always reveal that a false prophet sooner or later will contradict Christ himself. You don’t get more anti-Christ than that.
Remember it was Christ himself who said, “Man shall not live by bread alone, but by EVERY word that proceeds from the mouth of God.”