Some of the myths can be identified as later additions by the fact that they don't get mentioned before a certain point, by people who would have mentioned them. That's a lot of them, actually. The slaughter of the innocents, the clearing of the temple, the royal procession into Jerusalem—stuff like that would have been mentioned by a writer like Josephus, who mentions plenty of more trivial stuff along those lines, but they're not. The miracles, the virgin birth, etc., aren't mentioned by Paul, who would done so if they'd been part of the tradition in his day. In short, a lot of the events in the familiar Jesus narrative don't appear until near the end of the 1st century.
When you strip away all that stuff, what are you left with? There was a guy from Nazareth in Galilee who became an itinerant prophet figure and probably Messiah claimant for a short time before being crucified, after which his followers carried on his legacy and eventually became an identifiable movement within Judaism, then a separate group altogether as they and the more orthodox strains of Judaism developed a mutually antagonistic relationship. Most historians don't doubt that Jesus of Nazareth existed and got the ball rolling, but even two or three generations after his death, there's very little genuine biographical information available on him, so his followers fill in the gaps with a lot of mythic and midrashic content. Even then the historical Jesus was less important than the living Christ.
I think these discussions often end up with a false dichotomy, when it's entirely plausible both that Jesus was a real guy and that very little of the familiar narrative about his life is factual. The ancient biographies of kings and emperors are similarly mythologized, and those are people for whom there were good records by the standards of the day. If Jesus was really a poor fellow from Nazareth, it would be astonishing if we had any real biographical data on him. At the same time, we ought to be very careful about assuming that poor people from dinky little villages never existed just because there's no real documentary evidence on them. It does make them problematic as historical figures in the academic sense, though.
