Here are some general guidelines that may help:
TV set made before 1998 was a traditional "analog" set. If you bought a big-screen, projection TV between 1998 and 2004, it may have a built-in digital tuner inside. But chances aren't great. Only a limited percentage of projection TV sets (and generally only those 42 inches in diameter or larger) included digital tuners before 2004.
If you purchased a new TV set since 2004, your chances of having a built-in digital tuner improve dramatically. Starting in 2004, many of the TV sets sold at popular electronics stores feature digital tuners that will work after February 17, 2009. But it's not a sure thing. Even some of the newer TV sets are purely display monitors that lack the internal circuitry needed to pick up digital broadcasts. Usually these sets have been advertised as "HD-ready" or "HDTV monitor" sets. That means they can display digital and high-definition signals, but they need help getting those signals in the first place. You'll still need a special converter or a cable TV connection.