How Can I 'know for certain'?

As I studied Genesis 15 I was fascinated by Abraham asking The Lord how he could know for certain the the promises he had been given would come to pass.  I always feel terrible when I doubt what The Lord says, but here the Father of the Faithful questioned our Lord. I thought I should step away in case The Lord 'took him out'.  Instead I continued to read and found that God actually answered him and said 'know for certain' that these things will come to pass.  Then as I studied each of the promises I saw that they actually did happen. Just as he had said they would.

Now Abraham did not live to see these things happen. He walked about on the promised land but did not own any of it except enough to bury his dead.  However his descendants were given it many years later.
He did see a son from his own body, the result of a miracle in two very old bodies. And that was enough for Abraham.  He believed God and it was credited to him as righteousness. If Abraham questioned God and is remembered today as faithful, what can I learn from this passage?

My take away

I should care enough to find out what God has planned for me by reading his word and asking him what he wants to accomplish through my life. I should not be afraid to ask questions. It would be foolish to be disrespectful or accusing. But I see from the example of a man God called his friend that I can ask and that I can 'know for certain'.