A friend of mine got called a "posh tw*t" (in the nicest possible way) by her new friends on a writing course because she prefers sushi to pizza. Apparently that's one way to tell.
Another tells me that it all depends on where you keep your ketchup (in fridge is posh, in cupboard is not).
Or it might be according to how you order your bacon. Laura Lockington (author of the sublime 'Cupboard Love') said her grocer judged someone's poshness by the way they ordered their bacon. "If a woman asks for streaky, I call her dear. If she asks for back, I call her madam."
PS If you like your bacon in a sandwich with ketchup - that's better than classy: it's classless. Simply tops.