When evaluating free information, focus on your AUTHORITY to make sure your article contains citable information. Ask yourself the following questions:
Always keep in mind that online there is less editorial control over content. So you should always prefer the original author of the language, and check the document in which it was originally written. Watch out for sources that may paraphrase the original language.
If you find a quotation that you like, but cannot verify the authority of the article or source, use a search engine like Google Scholar and Google Books using the relevant language. It could be that the citation was quoted from an authoritative source which had not been credited. And even if it had been credited, make sure to check the original language.