Mathematicians must be trained for years to verify the abstract theorems of algebraic topology. So we should not object if monks must be trained for years to verify the subtle insights of Buddhist philosophy. As in any science, the practitioner must undergo years of discipline and training in order to understand and experimentally verify the propositions of the science. In principle, however, we all have the potential to verify the claims made by spiritual science, provided we awaken and train our faculty of spiritual discernment through appropriate disciplines. The fact that a non-physical form of knowledge is private does not imply that it can not be subject to independent verification.
Private knowledge can be universal intersubjective knowledge.
For example, although mathematical knowledge is verified in a private insight, it is one of the most universally valid forms of knowledge we have. The truth of a theorem is not a matter of opinion and does not depend upon personal or even cultural conditioning. Mathematics, therefore, gives an excellent example of private knowledge that is nonetheless universal in nature. It should be pointed out that even our perceptions of the physical world are ultimately private. Our knowledge of the physical world is highly individualized.