How do I go about analysing a sources that are cartoons in an exam?

The thing to remember when analysing historical sources is that there is a structure to follow: 

Firstly you want to make sure you look at the image very clearly. This might seem very obvious, but you would be surprised at the number of things you miss under pressure. Focus particularly on the byline or the description that mentions where the cartoon is from. The next thing you want to do is try and understand what the cartoon is about. The better you do, the easier it is. However, DON'T get hung up on this. The point of this question is that they want to evaluate the source and its value to the person/people in question (will usually be mentioned in the question). 

The way that you answer that question is by looking at the information that you are given. Is the cartoon from a reputable source? Do we know where it's from? Is there anything mentioned about the author and his knowledge on the topic? You can also mention if there might be any particular biases that you can see which might affect how reliable this source is.