There are many
resources for entering a screenwriting contest. You can look
at the contest's website, read about the winners, and should you
not be sure about the authenticity of the contest, you may even
contact some of the winners.
The cautionary note here is to
make sure you are entering a valid contest and do all you can to
research the contest before entering. The website will give
you an idea.
One hard copy book for a starter
resource, which sometimes is included free with your screenwriting
software is Erik Joseph's How
To Enter Screenplay Contests and Win! Some of the
contests listed here are not available online, are out of date, or
simply no longer exist. Still, this ebook is great for a
starter resource because it categorizes the contests, such as
which ones accept only horror/thriller, or comedy, or ethnic, etc.