Oh ok.
Well to determine if something is falsifiable, you have to be able to devise a test that could show that it could be wrong.
To do that for Determinism, you need to define exactly what constitutes evidence that shows Determinism is not causing an effect: namely that effects are occurring for certain without cause.
This is problematic because the intention here is to not only prove a negative, but also to prove a negative in a general case rather than just a specific one. I suppose you could try, for example, to find certain circumstances where none of the fundamental forces show their expected effects, and you’d have evidence that in that specific case, Determinism doesn’t model what’s going on. But for the general case you’d need to show the fundamental forces don’t work anywhere - despite the evidence being overwhelmingly strong to support the theory that they do operate everywhere we test them. Determinism is getting proven all the time, even as we speak, but that’s not to say it’s impossible to prove it isn’t.
There could be proposed a new theory that supercedes Determinism, which would partially disprove it in the same way that general relativity disproves that time and space are absolutes, even though in everyday conditions you can explain things very accurately even if you assume they are absolutes.
But if you wanted to prove that Free Will was going on in special cases like the mind, as a better explanation than Determinism, you would have to prove that, given a reason to choose one way over another, the outcome was no greater than random. Experiments show that not only can choices be influenced, the illusion that you were making the choices yourself remains in tact. You could try and trick this experiment, and choose as randomly as you can, in spite of having a reason to choose one way over another, but then you would also have to prove your impetus to trick was similarly not caused by prior conditions to any degree more than random chance - because your impetus to trick becomes your new reason to choose how you do. So Determinism catches everything here.
Basically, even if something superceded Determinism, it wouldn’t be Free Will to any degree, and if indeterminacy was going on in specific circumstances alongside Determinism, it won’t falsify Determinism. But that’s not to say you couldn’t find evidence that Indeterminacy was actually going on everywhere and the seeming Determinism was all an illusion.
So given all of the above, it should be clear that it’s perfectly possible to falsify Determinism in certain ways, but even if you did you would not be proving Free Will in its stead - to any degree. It would be some “God-of-the-gaps” argument to try and say, given any lack of Determinism modelling what’s going on, that we can safely fall back on a predecessor model instead. That would be fallacious, resembling the False Dilemma fallacy.
This is why, given the perfect falsifiability of Determinism as I’ve just demonstrated, I only give credence to indeterminacy in its stead, and even then only in specific situations alongside Determinism. If Determinism ever is falsifiable, it will be to yield to the next evolution up from Determinism, whatever that may be, but it won’t be a step back down to Free Will to any degree.