I think the proof is that we're still alive.
And 10 seconds later, we're still alive.
An astronomically tiny fraction of possible worlds would result in us being alive at all.
IMHO, we have total free will, but we can only be conscious of a world where we're not dead. That eliminates most of them. I don't know what further proof we actually need. If it's true, it will become more and more obvious over time as you outlive your peers, then outlive every other living thing on the planet. If not... well, I would have no way to prove that you didn't live forever (in your timeline). Best of luck to us, lol.
[edit] also, just back to the original topic... if the point of this universe algorithm is to monte carlo stuff, like compare a billion versions of the stock market on each of a billion planets, then each run is randomized and free will is a requirement. No individual timeline is pre-baked because the randomness has to go down to the quantum level to make it not be deterministic. If our universe were deterministic, there would be no reason to run the sim.
[edit 2] this is what I've thought for about 15 years, but I just pulled the above summary completely out of my ass. So you won't know until your brain is conscious in a tank orbiting Saturn 200 years from now, and Farrah Fawcett wakes you up and you go... "That guy from HN was right!"