We obviously cannot know. However those who claim to "know" of a will outside of our own require proof. If they cannot show proof then I cannot believe them. Simple.
If a man comes up to me in the street and insists that there is a flying dragon orbiting earth I would ask for proof, if he cannot give it to me, then I consider him crazy, I am sure you would too, so tell me how is the belief in God any different? A lie, just because it is believed by many, does not make it any more than a popular lie.