If he's coming from a faith perspective, I think he's asking if we knew where the canon of scripture came from, and if we research what the bible says (teaches) as opposed to listening to someone's interpretation of it.
If he's from a non-faith perspective, then maybe he's questioning the history of the bible and if we are aware of what it teaches as compared to accepted theory or fact.
I'll take the former position and state what I believe. The Old Testament are the Jew's scripture. Being a Christian, Christ confirmed them, so I have no issue with the right books being in place there. In other words, if I doubt the Old Testament, then the real issue is actually larger than what should be included in the canon. It's more of an issue of the person of Christ, but that's another topic.
It's the new testament that I am more uncertain about, but church councils aside, I do have some Old testament and the history stated in the book of Acts to help me there, too.
The Old Testament shown types and shadows of things to come with the feast days, the priesthood and the tabernacle objects. All pointed to Christ and His redemptive work. If we notice a common theme of 4 colors to the materials to make the curtains, we see 4 natures of Christ ---- these four natures are exactly (if one wants to say by "coincidence") the same portrayal the 4 gospels have, each one looking at Christ from that facet, i.e. his royal, eternal, servant and human (Perfect man) sides. The book of Acts, supposedly written by Luke, is historical specifically focuses in on the apostle Paul by the book's end as if to see his crescendoing importance. So, what do we have? epistles written mostly by Paul --- two different authors agreeing on this --- and even Peter in one of his epistles testifies about Paul. The rest of the epistles, as few as there are, came from men who were with the Lord directly. So these are first generation apostles and not a few later who could contort the gospel.
Lastly, there is a lot about the book of Revelation that they were not sure if it should be included in canon or not. But the undeniable connection with the book of Daniel and what Jesus said in Matthew 24-25 for example shows it is merely an extrapolation of what is already in the scripture but just not placed in one book all together.