Ideally, the prosecutor bears the burden of proof. We generally shouldn't impose systems that require defendants to prove a negative. I recognize that reality does not necessarily match this ideal.
It's ultimately up to juries to decide whether a defendant's assertion that evidence is fake is enough to constitute reasonable doubt in the absence of hard evidence for it. I imagine that's going to be very context-dependent. It would probably work if I was accused of this, with no history of anything like this, versus a guy who does this frequently, posts videos of himself doing it regularly, and never gave any indication they're fake until he got in trouble.
It's ultimately up to juries to decide whether a defendant's assertion that evidence is fake is enough to constitute reasonable doubt in the absence of hard evidence for it. I imagine that's going to be very context-dependent. It would probably work if I was accused of this, with no history of anything like this, versus a guy who does this frequently, posts videos of himself doing it regularly, and never gave any indication they're fake until he got in trouble.