I think an important question to ask is what "free will" actually is. I find people throw the term around without necessarily having a concrete definition for what they personally mean. When I use the term free will, I am saying that the the choices a sapient being makes would somehow be independent from all of the variables within the natural Universe. If it is dependent on natural variables (whether we know of them or not) this makes it deterministic, simply regular reactions to an unimaginably large domino effect.

The only hypothetical test I could conceive of would be if someone were able to rewind literally all variables of the Universe to a point in time just before decisions were made by another being, and doing this countless times and seeing if the outcome ever changes. I'd consider this to be entirely untestable, and as such I do not entertain the idea. For now I hold to what seems to be demonstrably true, which is that everything seems to operate and behave based on their properties, and react based on other variables which interact with them. With enough variables, we can make models to accurately predict the world around us, I do not expect our will to be any different.