In many cases, particularly those involving the Stetson family (the one I work on mostly) the Find A Grave information is a repeat of what O.F. Stetson and Mr. Barry gathered and which is found in "The Descendants of Cornet Robert Stetson."
Sometimes they give the citation, but usually the only giveaway is that I have the book open to the correct page, etc. and see where they got the information. FAG is good for getting parent's information on people where the Stetsons didn't record much of that information.
However, I was able to track down and actually visit some of the Stetson family sites in Maine and it started with following the family trail thru FAG.