Link building has always been one of the most abused SEO strategies out there. For this reason, many search engines have developed filters to check out the black-hat link building schemes. Some signs that indicate this behavior are:
Sudden break out of links. Links from site’s with bad reputations. Signs that you are selling links to your website. Any other signs which indicate that your link growth is not natural and is for the sole purpose of improving the site’s SEO.