One of the more challenging aspects is determining what is common knowledge, and each discipline has developed an accepted definition of what is considered 'common knowledge' for that area.
A question you can ask yourself when thinking about if something is 'common knowledge' according to Dr. Hanson (2011, in the video below) is: did you FIND this information or did you THINK it up?
A generally accepted definition is if the information could be easily found in 4 or more sources, it could be considered common knowledge.
This being said: if you are ever in doubt - cite it.
Examples of information that does not require citation:
Examples of information that should be cited: