Character A: Sounds and comes across as lower class, mostly, but with middle class overtones in everyday speech (accent and word choice). A bit of a chameleon.
Character B: very posh, attended Eton/Harrow/Oxbridge, very immersed in that kind of upperclass world.
During a rather intense conversation between A and B, I need B to catch on that A went to a rather posh, though not top tier, public school. (Which one does not matter). I need B to catch a reference, slang word, or something of the sort during the conversation that lets him deduce A's more posh background.
Thing is, I need it to be something that someone who has not attended such a school would not know. A bit of specialized, insider knowledge, that you don't just pick up by living in the UK [or through the Harry Potter books ;)].
I have googled a number of variations on 'british public school', 'customs', 'slang', etc, but I'm getting either information specific to a single school, or information that is well known publicly and would not be seen as an indicator of Character A's past.
(Alternately, would it be plausible for character B to know something about customs/slang of another school than his own, since he is swimming in the same pool as a lot of other public school pupils?)
EDITED TO ADD: Character A has worked hard to lose any indication of any upper-middle class/upper class characteristics, and left the school in question a few years at least before finishing. Ch. A DOES NOT WANT people to know about his background, is the idea. That's why I need something to jump out at Character B.