Cockney rhyming slang for "I should say so!"
Some rhyming slang doesn't make use of the divorced word convention, (as in 'apples' = 'stairs' because of the 'apples and pears' connection).
no, 'how do you like them apples' is American. Nothing to do with pears or stairs. I imagine a street trader asking what you think of his produce; but that's just a guess.
It's actually "I should cocoa" and originates from Tony Hancock's radio sidekick Sid James (who wasn't a cockney - he was a South African). Cocoa was a popular drink among the working class East Londoners and it's just a case of replacing two syllables with something everyone is (or was) familiar with.