I think it’s a fun question. I’d argue that sanity doesn’t exist, as the concept of sanity is constructed entirely of opinions–if a concept is made up of opinions, then the concept itself is an opinion, and is therefore entirely hypothetical.
How do you know you’re in your right mind?
I don’t.
I take it for what I consider it to be: a variable mix between a social contract (people agreeing on what is right and wrong (edit: healthy/not healthy), like with every single other thing going on in any society, whether I agree or not with them is not the point) and some common sense (what it means to be healthy (what sanity is referring to as far as I know) without arguing about abstract notions, like one would try to explain it to a young child maybe?)
So, really for me it’s an ad-hoc definition that has changed many times already, and will change again but it’s also something obvious that I would describe as being able to live among other people not in a perpetual state of anxiety… which may also incidentally imply I would not qualify as the best example of a sane person, who knows?