Re: brevity
You can go to
www.scriptsales.com. Call the production companies and agencies that are listed on there (legit, mainstream companies).
But if you are going to be paranoid, I'll describe what happens behind the scenes.
A script that is submitted to a prodco or agency is read by script readers/interns. They write a one page synopsis of your script, whether or not the producers should pass or consider it, and whether or not the producers should pass or consider the writer.
Often your script will be read by several script readers. They are not on the payrolls of the companies general, so there is no proof that any of them worked there.
So it is quite possible that someone will be inspired by your work. But that's true of all levels of the game. I can watch the movie pretty woman, decide I like it, and write my own script about a man falling for a hooker. The deal is, ideas are not copyrightable/protectable. The original expression of them is, which means I can't copy pretty women. My own script about a hooker has to be my own work.
As for someone "stealing your script" that won't happen for three reasons. It is assumed you know about registration, and script readers consider most scripts sent their way garbage. Just look at the comments of some of the people on this forum towards Zoetrope, or look at the things Simon has said. As for the producers, they can't write, so there is no way they'd take a writers work and simply stamp their names on it.