If that same person said they had mathematical evidence that proved he could see Saturn with his naked eye, I don't think I'm out of line for asking to see that data.
Well yeah - if someone could actually see the moons, they'd likely be able to produce evidence pretty easily (or at least they'd conciously know they're lying). You could ask them to say, for example, whether the moons are currently aligned or whatever. The positive side of this is really easy to resolve - if you find a dead Sasquatch, that kind of resolves things.
It's the flip side - "can you prove nobody can see the moons?" that's more the question here. I mean, you can give an explanation of why this would be very unexpected based on normal human visual acuity and how eyes work. And you can say that you've tested lots of humans and none of them can see the moons. But you can't stamp out the possibility that SOMEONE can tell the difference, just like you can't eliminate the possiblity of Sasquatches just based on never having seen one.
To be clear though, the presence of this possibility is not usually what's being argued by the "other" side. Rather, they're saying that the differences are large, reasonably easily heard, and attributable to phenomenon that don't make sense or they don't understand. It's not like they're saying "there MIGHT be Sasquatches". It'd be like if 1/3rd of the people in every discussion claimed Sasquatches were common and they saw them every day. And when questioned further, it really sounds like they're just seeing a dog - but they get very defensive when you mention dogs. Meanwhile, any time you've gone out to meet these people, they've been unable to show you a Sasquatch (time and time again) - but have pointed out several dogs.
You could imagine how it would get tiring being a "Sasquatch denier" and having this discussion often - so I don't envy people who are involved in, say, making audio equipment, and I don't fault them for being a bit chippy.