It is very hard to prove in a court of law that someone has stolen your idea for a script. Theft does happen, but usually it's just a case of two people thinking of the same idea at the same time. But there are ways to help protect your ideas, and you should take advantage of them.

To protect your work, contact the Writer's Guild of America, West at www.wga.org or Writer's Guild of America, East at www.wgaeast.org and you'll find information to easily register your script with the Guild. At this writing, the fee is just $22, and it's good for five years with the option to renew. You can register by mail, online, or directly through Final Draft. It is not necessary to be a member of the Guild. This is convenient peace of mind at a low cost, so take advantage of it!