I certainly have never studied philosophy formally in any fashion, but I have, however, had many lengthy conversations on multiple occasions on this subject, so perhaps my thoughts might be of some interest or useful in hearing another perspective.
I've never heard a convincing argument from any of my friends as to how one could possibly ever measure the perceptual difference between a creature whose destiny is determined and one who's destiny is not (possesses free will).
As soon as a creature begins to believe its actions are of its own accord, why be so quick to eliminate the idea that its perception/thought of controlling its movements and actions are indeed not a product of determinism itself? As for such a creature, if it were truly the product of a deterministic process, then all of its thoughts from then on until eternity would be in a sense inevitable, somewhat violating the idea of choosing thoughts and/or actions. Among such thoughts in the creature's head would be ones that gave the creature a sensation of choice, a brain state whereby it had the feel of choice, despite meanwhile remaining completely determined. (This type of thinking always brings to mind the concept of mistrusting a Boltzmann Brain by default, for its thoughts and perceptions are never genuinely conceived, but rather imposed on the organism.
As for a creature with free will, it would also feel as though it were in control of its actions by definition and thus would feel similar to a deterministic creature whose configuration, via determinism, created a state emulating the perception of free will.
Despite this line of reasoning always having felt recursive and leaving much to be desired, I've never heard an argument that has felt more satisfying in explaining just how a deterministic creature could distinguish itself from a creature endowed with free will.