There are a number of arguments which aim to prove the impossibility of free will.
The Standard Argument (incorporating the Determinism Objection and the Randomness Objection) is well known and powerful, although subject to a variety of criticisms The Standard Argument.
I seek here to provide an argument immediately testable via personal investigation.
NOTE: At the time of this edit - 24th August 2021 - no answers have provided detailed endorsement or rebuttal.
Is this argument sound?:
In order for an act to be voluntary, a person must decide to perform it.
A decision is an act. Therefore, in order for a decision to be voluntary, a person must decide to decide it.
This leads to an infinite regress of prior decisions, in which any voluntary decision must be the result of an infinite chain of prior decisions.
Insofar as free will requires the ability to make voluntary decisions, free will is impossible.
This argument constitutes a rephrasing of an old dilemma, but it remains useful here as a specific set of propositions against which to argue.