What to use to determine the validity of a subjective experience?
I think Ken Wilbur answers this question by saying that the 4 quadrants must be fulfilled in every quadrant. I think this is sort of like saying that there are natural laws.
How are the natural laws discovered? In the external or physical realm there is the scientific methodology that sets up repeatable conditions and then measures things as they are put through their paces. Of course, you come to the Quantum problem where in the thing that does the measuring affects the thing that is measured and therefore the thing measured is not the thing in and of itself.

The buddhist view is that all things are interdependent and so appearances are illusions created by the mind. This doesn't mean that you can't operate in the world because everything dissolves into fairy dust, but it does mean that if you say something doesn't change and this is the way it is and then make up laws about it or base your beliefs on that thing that you think will not change then you will find your belief eventually washed away by the winds of change.