Psalm 9:10 (NKJV)
And those who know Your name will put their trust in You; For You, Lord , have not forsaken those who seek You.
I was thinking about this verse this morning and I thought about the famous question asked above. Paul in Romans 1:20 says that God's creation clearly testifies of a Creator showing us his power and attributes. So we know there is a God but how would someone who had never heard the Gospel ever get saved? This verse in Psalm 9:10 tells that those who seek Him will never be forsaken. Our problem is not how can we know God but rather that we do not act upon what is already known about Him!
Jeremiah 29:13 (NKJV)
And you will seek Me and find Me, when you search for Me with all your heart.