Rocket launches are notorious for not making deadlines. Especially the first launch of a new type (ISS launches, interestingly enough, have a very narrow window, usually a minute or three, so they eventually get to the point where they can launch on time).
Therefore any date given today, for a date in the future is utterly meaningless.
SpaceX will undoubtedly make a BIG deal about a Falcon Heavy initial launch. Of course, you will be able to check the SpaceX home page.
Generally, the most reliable source for launch info that I have found is at SpaceFlight Now's launch tracking page. Which as of this writing, does not even have a date listed for a Falcon Heavy launch.
Amendum: The SpaceFlight Now page, as of February 2015, estimates a "3rd Quarter 2015" launch for the Falcon Heavy demo.