Automatic translations are frowned upon in Spanish Wikipedia as you may imagine. However, I find them to be more or less common, especially in articles with few editions.
They're easy to spot if they have obvious grammar errors, of course. But sometimes the choice of words and expressions may reveal that an article is (or was originally) an automatic translation. For example, if an article says that is some place "el español es hablado" (a direct translation of "Spanish is spoken") where "se habla español" feels much more natural for Spanish speakers, chances are that it was in fact automatically translated.
A rule of thumb would be to choose articles on general topics or topics well-known by Spanish speakers, because they will likely have been edited and supervised by many different authors. You can also try to go for articles that are highly rated by the community in terms of quality (artículos buenos / artículos destacados).