Echoing the suggestion to start tracking and counting instances of the spell. If something is up, it’ll show up in your numbers. If you wind up seeing roughly the right probability, you can rest easy knowing that either (a) things are exactly as they should be, or (b) the Universe knew you were testing and skewed the RNG.
On second thought, maybe that second explanation isn’t cause to rest easy.
Edit: I am going to toot my own horn, here.
This is what I am talking about. In the time since that thread went up, my tribute rate has remained constant – and below expectation. But because I am tracking everything, I know it’s low and don’t have to rely on infamously shaky human memory.