Weren't they written by different men, in ancient languages that are/were interpreted differently? How do you know which ones are original?
The source documents we have for Biblical texts have a tremoundously high amount of agreement with each other. It's not about knowing which ones are original. It's about knowing that we have accurate and useful translations from those source documents.
Authorship is a large issue unto itself, and probably should be the subject of a different thread.
As Baldie pointed out in 16, the manuscripts we have are numerous and in agreement. IIRC, there were 25,000 New Testament manuscripts in the early 70s when Josh McDowell wrote "More than a Carpenter." If the manuscripts that the NT are drawn from are not reliable, neither is any of ancient history. There are only five manuscripts of Julius Caesar's autobiography, for instance (again, IIRC).