First, which God? Read a little further in the Pensees and you will find that he suggests reinforcing a tentative belief by "having masses said, taking the holy water" which makes it clear that his wager is not a bet on God, but a bet on Roman Catholic doctrine. Let me ask you, would I be wagering correctly if I chose to be a Unitarian, a Jehovah's Witness, or a Jew?
Second, if there is anything an omniscient God should know, it's the contents of the human heart. Do you not think God could tell the difference between true belief and a belief concocted for the purposes of winning the "wager"??
I ask the same question of atheist. Which God do you know does not exist?
The creative principle of the universe; there is none? The source of your absolute truths; there are none?...
Often, in my experience when an atheist describes the God they don't believe in, my response is "I don't believe in that God either."