Check out this wonderful essay by Princeton graduate student Benjamin Schmidt: The Foreign Language of 'Mad Men'.
Using modern linguistic analysis tools, such as Google's Ngram analysis, Schmidt studies the language used in shows such as Mad Men, trying to see how authentic the language is.
Mostly it is quite accurate, but there are slip ups, some of them glaring:
There are scores of idioms that are strikingly modern. "Feel good about," "match made in heaven," "tough act to follow," "make eye contact," "fantasize about"; all are at least tenfold more common today than in Mad Men's times. Any of these individually might be perfectly plausible; but for "feel good about," for example, to be said four separate times over the course of the show by several different characters is extraordinarily unlikely.
But other anachronisms are extremely subtle:
What seems to be the most ubiquitous mistake in Mad Men is so frequent as to be invisible: the phrase "I need to." Modern scripts set in 1960s, including Mad Men, use it constantly: it's about as frequent as everyday words like "good," "between," or "most." But to say "I need to" so much is a surprisingly modern practice: books, television shows, and movies from the 1960s use it at least ten times less often, and many never use it all. Sixties dialogue written back then used "ought to" far more often than modern imitators do.
It's a very interesting approach, combining computer analysis with old-fashioned linguistic study, and is a fun read.
Special bonus Mad Men link: if you're revving up for Season 5, take this quick 7 minute refresher course on the first 4 seasons, courtesy of the Slate Audio/Visual Club!