No. Too many unknowns--I may simply disagree with, say, Tricky, simply on principle, or in fun. I may agree with Darat in order to keep my lucrative job. I may parrot things I don't understand, but which Dr. Adequate doesn't know enough about to call me out on.
Consistency is a clue, but not enough. If someone is internally inconsistent in his or her posting, I would suggest that they do not mean (or perhaps understand) all that they say. But they may believe some of it, or may believe all of it out of ignorance...
Bottom line, for me--no, there is no way to prove that you believe something. Nor, under most circumstances, is there any need to.