Read the screenplay from start to finish. Have the script fresh in your mind and know all the details. It may even be useful to re-read the script a couple of times so you are more familiar with it. If you are easily reading through it multiple times, that is an indication that it may be good.
Analyze the concept of the film. Ask yourself if the concept is original, easily marketable and full of emotion. The script must have a theme. Determine if the theme is important and one worth sharing. Also, the stakes must be high. If there is no drama, there is no story. The story must be logical and make sense within the rules setup for that story.
Analyze the structure of the film. First and foremost, the screenplay must be properly formatted with scene headings, action lines and dialogue. The screenplay must also have a three-act structure -- set-up, confrontation and resolution. Although some stories may not strictly follow the three-act structure, every story has a beginning, middle and end. There should be rising obstacles leading to the climax near the end of the film. The script must draw the reader in within the first 10 pages and keep the reader guessing throughout. The writing should be visual, "showing" (through the characters' actions and words) what is happening rather then "telling" (through the narrator's exposition). The script must also be of proper length, between 100 and 125 pages.
Analyze the individual scenes of the screenplay. Every scene must be important to the story and move the story forward. If a scene does not develop story or character, it should not be there. Like the script as a whole, each scene should have a beginning, middle and end. The pacing from scene to scene must flow and the reader should never be able to guess easily what is going to happen next.
Analyze the characters. The hero of the script must be likable. He should have wants and needs that must be satisfied and he should undergo change to achieve his goals. Characters should be multidimensional, with varying points of views, wants and fears. The antagonist must be stronger than the protagonist, making the hero’s journey more difficult.