Questions to ask:
Is there a description detailing the coverage and scope of the source? In books this may be found in introductions, forewords - even the dust jacket or back cover of the book may give you this information. In the case of an article this may be found in an abstract or other summary material found on the first page of the article or other descriptive information within the journal - or possibly where the article is indexed if it is found in an abstract or online index which also contains abstracts/summaries of the article. Good web sites will have this information - often it is found in the "About" link if not on the front page.
If it is a web site is there a print equivalent you can compare it to?
Is the content complete or are there obvious holes in the coverage?
If a web site, any signs that the site is still under construction? If it is it is not yet complete, so coverage may be lacking.
Is there the same level of coverage throughout the resource - the same depth of coverage?
Can you recognize any obvious omissions?