Open the script. A properly formatted script will have the page numbers in the upper right-hand corner of each page, except for the title page. Flip to the last page and read the number.
Count the pages by hand if there are no page numbers on the script. This can be time consuming if it is a feature film script or videogame script.
Read the script. If there are no page numbers on the script, or if you are looking at it online in HTML format, it may be difficult to accurately determine the script length. A general rule of thumb for film and television scripts is that one page equals one minute of screen time. This isn't always accurate, but can be a good indicator of the length of a script when it is not possible to count the pages by other means.