It seems to me that free will can be easily tested. The next time someone is getting brain surgery, just take a few minutes to perform the test. Sometimes the patient remains awake during brain surgery so he can report what functions are changing as the surgeon is poking around. So for example, when the surgeon electrically stimulates the language center of the brain, the patient might temporarily lose his ability to speak.
If the patient can speak normally despite having the speech center stimulated, then the patient has free will that can overcome the normal chain of cause and effect in the brain. If he can’t speak, then you have proven the brain is nothing but a moist and complicated machine and your life is a pointless series of miseries.
Talk about a stacked deck. What believer in free will has ever thought that you are free to choose no matter what? Why not just give people lobotomies to "prove" there's no free will?
Conversely, it's clear that if the believer in free will passed Adams' test, determinists would just say that the speech center didn't get enough electricity.
In the end, what the determinists have going for them is the axiom of causality. And what believers in free will have going for them is virtually all of our waking experience. Decades after first hearing both sides of this debate, I still choose ubiquitous introspection over a plausible a priori postulate.