The context of a cartoon is important. Most of the time, cartoons are attached to articles and usually draw upon a point contended by the writer of the article.
Does the cartoon support or oppose the article?
Does it focus on the past, present or future? Which aspect of the article does it relate to?
Does it add further information?
However, there are times when you have to analyse a cartoon alone, where it is not accompanying an article. In this case you will have to understand the background, the situation and the issue that is represented.