There is also internal evidence. Matthew was a former tax collector, so the fact that his book speaks of financial topics much more often than mark or Luke makes perfect sense. Luke was a physician, so the fact that he didn't mention the woman's condition getting worse under the care of physicians makes perfect sense. The book of John explicitly claims that its author was a disciple of Jesus. Mark is generally thought (by those who hold to the traditional authorship) to be a compilation of sermons that Peter gave in Rome, so the fact that it is the shortest of the four makes perfect sense.