Access the viewing guide on your cable box if you have one, visit the TV Listings section of the Internet Movie Database or the online TV Guide, buy a current copy of TV Weekly, or pay attention to TV and radio commercials to find TV specials that are airing soon in your area. TV specials are usually heavily promoted in advance, so the networks try to make them as easy to locate as possible to increase viewership.
Public transportation is another place to find advertisements for upcoming TV specials, especially in urban areas. Bus stops and the sides of the buses themselves often have advertisements for films and TV shows, and these often include specials that are airing soon. Billboards are another common place to see TV special advertisements.
Many TV shows advertise at movie theaters before the feature presentation begins. If a TV special has a large enough budget and is a big enough priority for the network airing it, an ad for the program may air in the pre-show presentation at the movie theater for a few weeks before the special occurs.
If you have a smart TV, it likely has a TV Listings option as well, even if you do not have a cable box hooked up to it. There is usually a button on the remote that brings up the local listings for the next few days and a search feature where you can look for upcoming specials.