Authority: Author’s credentials, area of expertise, publications, affiliation, etc. The authority of the source: Is it peer reviewed? Is it discipline-specific?
Objectivity: Is there evidence of bias?
Accuracy: Peer review, citations, references.
Currency: Is the information up to date? If an older source -- is it relevant to the historical moment you are researching?
Relevance: Can it help answer your research questions and/or generate new questions?