The guidelines talk about general ideas and concepts. They do not provide exact percentages and numbers so there is room to debate what is compliant and non-compliant.
Google has become more active in notifying websites via Webmaster Central that are breaking the guidelines. So you can check to see if there are any messages about possible issues with the site. Google is not notifying 100% of the time so this is not perfect.
If you are thinking about buying a website you should have a thorough website audit performed so you can make a more informed decision when negotiating the price.