Not remembering the title of a movie can be frustrating -- particularly if you only recall one clip. However, if you can recollect enough details about the one clip you viewed, there are a couple ways you can find out which movie it's from.
Recall any famous actors or actresses that were in the clip that you watched. Type their name into the search bar on The Internet Movie Database website and select the actor or actresses IMDb page. A filmography listing all the films they have starred in will appear. This is the easiest place to start when trying to figure out the name of a movie. While some actors and actresses have starred in a multitude of films, the search engine will cut down your selection to a handful of movies that your mystery clip could be from.
Attempt to determine roughly what year the clip you were watching was made in. Although it will be difficult to pinpoint the exact release year of the film, things such as the quality of the special effects, technology featured in the film and even the age of the actor or actress in the film can help narrow down the time frame and further narrow your selections. However, if you are watching a period-piece, ensure that you are distinguishing elements of production from props and setting devices featured in the film. Once you have determined the rough date of release, apply those parameters to the actor or actresses' IMDb filmography to futher narrow your selection.
Recall any lines from the clip you watched. If you remember a spoken line close to verbatim, you can quickly type the line or a portion of the line into an Internet search engine. The results should quickly tell you the name of the film it is referenced in.
Describe the clip to a clerk at a movie rental store. If they have been working there for a long time, they should be familiar with many movies and may be able to tell you which movie the scene is from. You could also try asking friends and family if they know which movie you're describing.
If you have narrowed your search down to just a few movies, go to YouTube and type in each title of the films you think the clip you remember is from. Watch trailers and clips from each potential film to discover if it fits your description of the style, story and details of clip you remember.