That is an excellent question and it just so happens I wrote something up a few months ago when I was discussing the criteria with
@Tiberius, so below is what I wrote.
The minimum criteria would be:
1. He had good character as exemplified by his qualities such as love, mercy, kindness, truth, justice, benevolence, gracious, merciful, righteous, forgiving, patient.
2. He believed he had been given a mission by God and did everything he could to see that it was carried out. He was completely successful before his death, and he accomplished everything that he set out to do.
3. He wrote much about God and God's purpose for humans both individually and collectively, or scriptures were written by others who spoke for him. He firmly believed that the work he was doing was for the Cause of God.
4. He had many followers while he was alive, and there are still millions who follow his teachings and gather in groups based on the religion he founded.
5. His followers have grown more numerous in recent times.
I said: "This is a starting point but there are other questions we would want to ask ourselves before we would be able to believe that a man was a true Messenger of God because that is a bold claim so there should be a lot of evidence to support such a claim."
Other criteria he would have to meet is that his religion could not contradict or be in opposition to any of the world religions that are already established and he could not talk down any of those religions and say his religion is the only true religion from God.