By denial. By negative knowledge. By knowing what God is not. This is the backbone of Orthodox theology: apophatic knowledge. The west always relied on the cataphatic proof, which is closer to our logic, but limited (just as out mental capacity is). Apophatic knowledge goes beyond that.
A good ex maple of the cataphatic approach is St. Thomas Aquinas' Summa Theologica. The other one is Catholic teaching on transubstantiation.
The Orthodox approach to God is best summarized in St. John of Damascus' Exact Exposition of the Orthodox Faith, Book I, Chapter III
That's near as fine a "definition" as we're likely to find. But it is of course totally and necessarily incomplete since Ο ΩΝ cannot be described. The apophatic theology of the Christian East is as effective as it gets when it comes to speaking about God. The finest expression of this is the Cappadocian comment, "I believe in God; God does not "exist".