I guess it depends a bit on how you came to consider the proposition to be tested, but I’m not sure how to formalize it.

I wouldn’t waste a moment’s attention in general to some random person proposing anything like this. But if someone like my mother or father, or a few of my close friends, suddenly came with a story like this (which, mark you, is quite different from the usual silliness), I would spend a couple of minutes doing a test before calling a psychiatrist. (Though I’d check the calendar first, in case it’s April 1st.)

Especially if I were about that age. I was nowhere near as bright and well-read rationalist!Harry at that age (nor am I now). I read a lot though, and I had a pretty clear idea of the distinction between fact and fiction, but I remember I just didn’t have enough practical experience to classify new things as likely true or false at a glance.

I remember at one time (between 8 and 11 years old) I was pondering the feasibility of traveling to Florida (I grew up in Eastern Europe) to check if Jules Verne’s “From the Earth to the Moon” was real or not, by asking the locals and looking for remains of the big gun. It wasn’t an easy test, so I concluded it wasn’t worth it. However, I also remember I did check if I had psychic powers by trying to guess cards and the like; that took less than two minutes.