Google looks for hundreds of patterns like: You are using the same or a small list of anchor text in a vast number of incoming links. Getting a lot of links from sites you link to, especially if either you or the other site fails to link to other websites as well. Links to and from sites with a relationship (like the same WHOIS owner), especially if there's no disclosed connection. Google is an accredited domain registrar, which means they have far more access to domain registration information than your average person. They check for whether or not the links you're receiving are actively penalized. They check to see if the websites linking to you have relevant content. The time that the backlink was first discovered. The author and quality of that blog post. These patterns are how Google can detect things like PBNs, hidden link schemes, and link exchanges.