1. In determinism everything that CAN happen MUST happen. This is because even though at some points there might appear several choices, actually there was only one choice, the predetermined choice. All the other choices were a priori impossible and only illusionary.

2. Refuting determinism and thus proving free will would require a scenario where there actually is more than one choice possible. For this we need an experiment yielding at least two different outcomes from the exact same preconditions.

3. Quantum mechanics did it. The double slit experiment allows for actual choices. Each run of the experiment is a survey conducted on the free will of individual particles.

4. Humans exist of a collection of quantum particles, constantly collapsing their wave functions and thus exerting their free will. Therefore humans have free will.

QED