I've never seen some consensus as to the definition of free will to the extent that it was testable. As in, "What experiment can we perform to show this thing

I've never seen some consensus as to the definition of free will to the extent that it was testable. As in, "What experiment can we perform to show this thing has free will?" or "Define free will in such a fashion that a universe with free will is notably and different from a universe without free will, in a manner we can detect." If we cannot do that, I am not even sure what the point is of discussing the concept. It is like some box we are told lies sealed in a distant continent, and we argue about what is in the box. Why? We'll never know, and the answer doesn't seem to change anything.