Excerpt from John Owen's Greater Catechism

Question 4: How do you know the Scriptures to be the Word of God?

Answer: By the testimony of God's Spirit, working faith in my heart to see the heavenly majesty and clear divine truth that shines in them.

Supporting Scriptures:

Matthew 16:17
John 16:13
I Thessalonians 2:13
I John 2:20, 5:6
Luke 24:32
I Corinthians 2:14
Hebrews 4:12
II Peter 1:19

Note: Other reason often given but not valid: the authority of the Church in choosing them (Roman Catholic, Orthodox)

Other reasons often given, valid but not definitive: their apostolic origin, the confirmation of archaelogy, prophecy, internal unity.