Begin watching the movie. With your pencil and paper handy, begin marking down things you think are appropriate for kids, and things you feel are not appropriate.
Note how many curse words are said in the movie. Movies with a 'G' rating should have zero curse words. Keep track so that when you write up your screening, you can give an exact number. Also, look for scenes that contain violence, nudity, drug use and other potentially upsetting material. Write down approximately how much time in the movie is spent with these scenes.
Write down any scary or intense portions of the movie and when they occur. Assign the movie a number from 1-10, which is a better indication of violence than a standard MPAA rating. A movie rated a 1 should be suitable for all audiences, while a 10 would be for very mature audiences only.
Decide on an age range that would be appropriate for the movie you are watching. You can do this by taking into account all of the movie factors that you noted above; whether there was cursing, violence or scary situations. The more of these scenes in the movie there are, the older the age range should be.
Write up a screening report that you can submit to kids' screening websites, post on a blog or social networking site, or just use for your own future reference for your children. See Resources for other sites that screen children's movies and accept screening reports.