There’s a basic approach to this using markov chains which works surprisingly well. Scott Aaronson once challenged some students to beat his algorithm — only one student could, who claimed he just “used his free will”. Human randomness isn’t so random. There’s a neat little writeup about it here: https://planetbanatt.net/articles/freewill.html

I like to think that this is a measurement of free will in the literal, naïve sense. It makes just as much sense as other definitions (the ability to take action independent of external cause) and it has the bonus of being quantifiable.

The only downside? A LOT of people get very mad at the implications.