Given that God is defined as a perfect being:
Prove A: There is a perfect being.
Assume ~A: There is no perfect being.
~A --> B: If there is no perfect being, then no flaws can be known.
~B: Flaws are known.
Hence, ~~A: by modus tollens.
Therefore, A: There is a perfect being.
(~B) is fairly certain. We know that, "bachelors are married," is a false proposition. We also arguably know other intangibles, such as, "Rape is morally wrong," where such a proposition posits moral realism. Hence, (~A --> B) is the key premise.
C.S. Lewis' famous analogy pointed out that one wouldn't call a line crooked, unless there is already knowledge of what a straight line looks like. It seems, then, that in order to know that something has a defect, or "flaw," that something perfect must be the standard for making such a determination.
One weakness of this argument is that it appears to only demonstrate the necessity of some abstract notion of a perfect being. In what sense does God interact with the universe, and with human beings in particular? Descartes' answer was that the idea of a perfect being had to have been caused by a perfect being, since a finite being (i.e. a human intellect) is unable to produce anything perfect. I'm not completely satisfied with this answer, though. One might object that the concept of a perfect being can produced by a human intellect, even if the reality of a perfect being cannot.
We might supplement Descartes' argument with an appeal to some form of general realism. There are certain intangible truths that are true independently of whether humans recognize them as true or not. Moreover, what is it that provides unity between the universality of some truth and the particular mind that knows it? The principles of one cannot be used to explain the principles of the other. If there is something that provides such unity, then presumably this would be something like the mind of God.