Green’s Dictionary of Slang

an interrog. phr. calling for agreement that something is worthy of praise or approval.

1937		J. Curtis You’re in the Racket, Too 42: How’s about that for an idea?
1967		P. Thomas Down These Mean Streets (1970) 13: A few weeks later Poppa got a job in an airplane factory. ‘How about that?’ he said happily.
1985		Eble Campus Sl. Oct. 6: how about that (it) – expression substituted for saying ‘My goodness’ or ‘You’re kidding?’.