No one can truly predict how well a movie will do at the box-office. However, people will gladly charge you for the information.
Anyone has the ability to speculate if a film has commercial potential.
Keep track of scripts that sell and their concepts. Compare that to the scripts that are greenlit. Compare that to the films (and their budgets) that are actually released (theatrically or to DVD originals). Track the amount of screens and the box-office (or units sold). This will give you a very good picture of what Hollywood looks for and how its goals perform in the long run. The performances of these films determine the trends and what Hollywood might produce in the future.
This information is available in
The Hollywood Reporter or
Daily Variety, but you have to read these papers daily to keep up on this info. You can read the trades online for free. Anyone seriously considering selling a script or a career in the film industry should be reading the trades.
If this requires too much work, simply see what movies are playing at the local multiplex and how they fare at the box-office.
Most any Hollywood exec or agent (the buyers and sellers) could tell you if a script idea has a logical chance of selling in the current marketplace. Paying someone to tell you the marketing potential of your screenplay is tantamount to paying someone to predict the weather for next week. Aside from there being no guarantee in the prediction of rain or sunshine, you are surrounded by free information (the same info the pro uses) to come to your own conclusion.
General rule of thumb, if Hollywood is making similar movies to the ones you're writing, then you're in the loop. If you're trying to re-invent the wheel, you'll have a harder time breaking through. (Most execs/agents/managers/producers are not interested in wheel reinvention.)
This takes some introspection, as there are writers who cannot discern the difference between their screenplay of a twelve-year-old transvestite who runs away to join the circus from a film like HARRY POTTER.
