Bart Ehrman, a New Testament scholar, has a methodology for deciding the probability of any part of the NT is fact or fiction.
We have to remember even with Ehrman’s methodology, there is still scant evidence of what happened a couple of thousand of years ago.
Ehrman uses three factors to judge the authenticity of stories in the NT. First, the earlier the writing was done, the more likely a story reflects something that actually happened. Again, it is not a guarantee.
Second, does the story come from multiple threads of story telling, or just one? If the story seemed to come from more than one source, the more likely it is accurate.
Third, and this one requires some sophisticated thinking, does the story provide the propaganda the story tellers wants to leave with the reader or not. Ironically, if the story does not provide the desired propaganda, the more likely it is to be accurate.
There are a couple of examples where these principles can be applied. One is the resurrection of Jesus. It is not included in the earliest account of the Jesus story nor does it have separate independent sources. And, in enhances the propaganda that Jesus is a God. These make the resurrection unlikely.
Another is are the statements attributed to Jesus proclaiming he is a god. They don’t appear in the earliest Gospels, only in the much later one, John. Jesus probably did not say this.
How Jesus Became God. Bart Ehrman