How do we know?

I guess this is a question that comes up over and over again. How do we know that God has spoken? I find two ideas put forward by believers.

1) God has revealed himself in his HOLY WORD!!!

I guess there are two questions that occur to me.
Which book? There are so many claiming to be God's Word, which one is right???
Even the Christian religion has many different bibles.

If I agree with which book does that mean once I read it I can fully understand God?
Funnily enough Christians even with the same bible can't agree what on what it says.

Conclusion: What believers mean when they talk about the Word of God is "believe what I tell you this book says and don't ask any questions,"

2) God's spirit has revealed the truth to all.

I guess the questions that spring to mind are.
Which spirit is actually God's?
Many people claim to hear the Spirit and they all disagree. Either God is very confused or some people are deluded.

The second question is a follow up: How do you tell the difference between God's spirit and others?

I guess when believers say the Spirit has told them they are actually saying "trust me I know what is best."

Overall I would say it all boils down to

"Don't question me!!"