There are ways to identify paid advertising links.
1. Contextual links to sites unrelated to the content, especially multiple links to the same site within an article. This is what identified and got the blog networks banned.
2. Links to poor quality sites on almost every page in a site. A natural link would be to a high quality site that already ranks well. Another issue with the blog networks.
3. Certain types of unnatural sidebar or footer links.
4. Regions of a page that are obviously reserved for advertising and contain advertising wording. This is common in the travel industry and a few other industries.
According to Google, selling advertising links is not a problem as long as the rel="nofollow" attribute is included in the link. An advertising link should be able to promote traffic, but not pass link juice.
Keep in mind that Google doesn't like anyone to make money on the Internet unless their name is Google. Over the past few years, they have destroyed billion of dollars of value on the web.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin