If someone says they experienced free will today, the claim is ludicrous without a definition.
As such it's on them to first state a complete hypothesis of what happened to them, because absent that it is not even possible to start disproving it, and so rejecting it out of hand is perfectly justified just as much as it's justified rejecting Russel's teapot.
> What is your benchmark for determinism?
That everything we have so far observed with any kind of precision has turned out to be predictable to our level of precision or to show no sign of deviating from randomness.
Determinism is also a falsifiable theory: All you need to disprove it is to show the existence of a type of events which are biased in ways which can't be explained in a mechanistic or by randomness.
The total lack of such examples, despite a very substantial amounts of claims, which always collapses under scrutiny, gives me as much confidence in determinism as in gravity.
Our understanding of gravity could be massively off too - I'd consider it far more likely that we'll need to revise our understanding of gravity than determinism, because a non-deterministic model would require a fundamental change in logic to accommodate another category of causality which can not be modelled as a computation over a combination of mechanistic determinism and randomness and without just introducing regress (the last part there is essential to rule out "God did it" or "we're in a simulation" answers, both of which just lifts the question one level up).
The first step in falsifying determinism is to actually find a way of determining whether or not an event fits that criteria to show that it's even logically possible for such events to exist. Just that would get your name in the history books. Of philosophy anyway. And if anyone could present a logically sound argument for how such events could exist, it would seriously shatter my world view. I'd love that anyway because it'd force us to rethink a whole lot of things about the world from scratch. Unfortunately I think I'm more likely to meet santa.
> Yet many believe they experience free will everyday and many don’t, because it’s a subjective experience.
People's subjective experience is not in question. I feel like I make free choices every day as well. I feel like I'm exercising one right now in choosing to answer you. That does not give me any reason to presume that feeling reflects the underlying processes. That subjective experience have no bearing on the issue.