Google can check the length of time that a link persists in Wikipedia. For pages that have high traffic, spam links get deleted very quickly. The SEO page is a good example. Try posting a link there - it will be gone in minutes, and you'll get a spam warning on your user page. Wikipedia is a fairly good map to knowledge on the Internet, often better than Google.
It depends on the topic, and I can assure you I know of plenty of topics at Wikipedia where spam and propaganda have been allowed to stay.
Right now, Wikipedia is a spammer's paradise. They are doing an awful job of policing the garbage and I expect the problem to get worse as more and more people recommend it as a useful or worthy resource.