Ask the right questions. Determine a screenplay's audience by asking yourself "Who is this trying to reach?" Picture the screenplay being made into a film, and then ask "What kinds of people will be attracted to this story?"
Determine the genre. Most genres are pretty well stocked with a target audience. If you are writing a romantic comedy your audience would be mostly chicks. If the screenplay is full of explosions and manly fist fights, a la action flick, then your audiences is probably going to be dudes.
Find the heart of the plot lines. Determine a screenplay's audience by really getting to the meat of the story and begin to make connections. When looking at a story of a young kid trying to overcome adversity, you would immediately make a connection to kids and teens.
Make a list of the people who may be interested in the story provided by the screenplay. Begin to write down what kinds of gender, age and other demographics that coincide with your list of audience members.
Question whether or not the script is geared toward a particular set of ideals. People are as different as movies are. There are horror flicks, which require little mind power to watch, and there are artsy movies that leave you scratching your head for weeks before the light bulb comes on. Read a screenplay carefully to grasp who the main characters are and what kind of audience would be attracted to those people.