How do I know the Bible is really God's Word? 

Many people will argue that the Bible is God's Word (we sometimes call this "inspired") by quoting 2 Tim 3:16 "All Scripture is God-breathed and is useful for teaching, rebuking, correcting and training in righteousness." However this is circular logic. We begin by assuming that the Bible IS God's word and then have it say that it is. This doesn't hold up. Fortunately we have a God that provided further proof of His Word. I am going to quote Josh McDowell from his book A Ready Defense. I will put my own comments in italics after he makes a point.

Instead of assuming the Bible is the Word of God, we can begin by demonstrating that the Scriptures are basically reliable and trustworthy historical documents. This is confirmed by applying the ordinary test of historical criticism to the Scripture. The Scriptures stand up to any test that we put normal historical documents through. In fact the Bible could stand up to more rigorous tests of historical accuracy than we need to put something in the history book. Society today is reticent to claim the Bible as accurate because of the implications that this would cause. Nevertheless, the Bible does withstand all standard tests as a reliable and trustworthy historical document.

Once it is established that the Bible is a valid historical record, the next point is realizing that Jesus Christ claimed to be the unique Son of God and He based this claim on His forthcoming resurrection from the dead. He also based His claim on miracles and His death. Both of which we find evidence for in historical documents.

Next we examine the evidence for the resurrection contained in this historic document and find that the arguments overwhelmingly support the contention that Christ has risen from the dead. If this is true, then He is the unique Son of God as He claimed to be. If He is indeed God, then He speaks with authority on all matters. If the Bible was just a regular trustworthy historic document there is enough evidence to back up Christ's claim. Over 500 people saw Christ risen from the dead. We believe things from other historic documents including David Bowie's death at the Alamo with less eyewitness accounts than this.

Since Jesus considered the Old Testament to be the Word of God (Matt 15:1-6, 5:17,18) and promised His disciples, who either wrote or had control over the writing of the New Testament books, that the Holy Spirit would bring all things back to their remembrance (John 14:26), we can insist, with sound and accurate logic, that the Bible is God's Word. This is not circular reasoning. It is establishing certain facts and basing conclusions on the sound logical outcome of these facts. The case for Christianity can be established by ordinary means of historical investigation. Does that make sense? Based on historical evidence alone Christ is the Son of God and therefore the Bible has to be God's Word.

Alright, back to me. Again that was Josh McDowell's book A Ready Defense copyright 1993. If you are interested in more arguments for the Bible's authority and historical accuracy, please pick up this book. The first 175 pages have to do with proving the different aspects of the Bible being God's Word.