How do I evaluate a historical source?

When evaluating a historical source, it can be helpful to consider the following questions:

When is this source from?

Where does this source come from?

Why was this source made?

By whom was this source made? 

If you always approach a historical source by considering these questions, it will become clear what its qualities and limitations are. For instance, if your source is a Nazi poster from 1941 then it follows that it is most probably a form of propaganda. Now that the motive behind the source is clear, you are able to see that this poster is valuable in showing the Nazi ideals of the time, but that it does not reflect the general public's way of thinking, or reveal their reaction to such posters. A historical source can become more valuable when confronted with another one so make sure you collect as many sources as you can to construct an argument!