You cant, thats the point. And it is true of concepts like an omnipotent being (how can you test if a being is really omnipotent?) or Lastthursdayism (which claims that the world was created last Thursday, but with the appearance (including history, memories,
) of an older world).
Likewise, if you claim the existence of a being which cannot be seen, heard, smelt, and cannot be detected by any (reliable) means, then the existence of such a being, as well as its properties (its shape, gender, color,
) cannot be tested, and your claim is unfalsifiable.
But this is not enough: if I'm not mistaken, the Bible claims that God is omniscient. how would you test it?