Even in the Muggle world, you're unlikely to get through a full day without hearing a Harry Potter reference of some kind. Like the fact I just called us all Muggles, for example. Or when people describe grand old buildings as looking like Hogwarts, whether or not they actually do. But you can spot a true Harry Potter fan when they make reference to such oddly specific moments in Harry Potter that you just know they must have read the books 367 times each.
Any old no-maj can decipher a reference about "pulling a Neville Longbottom" when you mean that someone suddenly got unexpectedly hot. It doesn't take an Arithmancer to figure out the references sprinkled throughout countless TV shows to fictional wizards called "Parry Bladder" who go to schools with names like "Pigblisters." No points to Gryffindor for spotting any of those.
No, I'm talking about those strangely specific references to minor incidents and characters that only the most devoted Potterhead will be able to piece together. If you can manage to slip any of these super-specific Harry Potter references into a conversation, then you deserve an Outstanding in your N.E.W.T. exam for Potter fandom — and if you notice anybody else mentioning them, then you know you've found a friend for life.