hello KB, the free way: use IMDB. But this will only show you distributed shows and not titles that have been trademarketed but never used.
A better, but more expensive way: http://www.thomson-thomson.com/ is the leader in title search. This is what your attorney will likely use if requested, but you can do it directly and save the attorney's fees. However, we don't spend $1 in title search, copyrights, trademarks, etc unless we are distributing the film/tv show directly. If your goal is to sell the project to a major network or to a distributor, they will likely run the title search and change the title to something they want. So we usually don't bother about the 'perfect' title since this will likely change 10 times before it airs.