To help understand an historical source, it is important consider:
Who created it? This could include finding out their: name, age, gender, nationality, occupation, religious beliefs and political allegiances.
What type of evidence is it (e.g. a document, an artefact, a photograph etc.)?
Where was it created? Was it found in the same place or had it moved?
When was it created?
Why was it created - what was its purpose? A source is biased if it only contains one person's opinion/viewpoint on a topic.