Look at the material nature of the source. Knowing whether it was printed, published or hand-written, the quality and type of paper or binding material, and the typeface or style of handwriting will provide clues into the intentions of the author and the context within which it was written.
Learn about the creator. There may be information about the creator within the text (where they were from, there profession, age, etc.) or the name can be looked up in historical records. If well known, there may be published biographical materials that can be referenced.
Check the time and place of the creator in relation to the event. Chances are, the closer the creator was to the event (in time and space), the more accurate the information will be.
Consider the intended message. Do not assume that you have pure facts in front of you. Sources will likely be biased and the intention of the author in creating this text should be judged critically.
Consider the intended audience. Knowing whether the author was creating a message of persuasion for a large audience or was writing a private letter to someone will help you judge the intentions, the accuracy, the honesty and the emotion of the message.
Crosscheck the document with other documents surrounding the time and event. Find out if other sources support or contradict the information.
Make conclusions about new or supplemental information the document provides in respect to the event, time or place.