Google states that “buying or selling links that pass PageRank can dilute the quality of search results. Participating in link schemes (https://support.google.com/webmasters/answer/66736 ) violates the Googles Webmaster Guidelines ( https://support.google.com/webmasters/answer/35769 ) and can negatively impact a site's ranking in search results.” Google isn't great at detecting all of these links it would appear...
Note if you are buying or selling links for your client, reporting your competitors may cause your client to be scrutinised by GOOG as well . :)
These paid / sponsored links are in my experience most often a paid blog post on a 3rd party site linking back to their site ( = paying someone to place a blog post / asking someone to review a service on their site in return for a freebie like hXXp://www. runoutofwomb.com/2016/01/review-four-seasons-hampshire-britains.html )
These posts sometimes have some or all of the following characteristics:
1. they look out of place e.g. a post on “Digital Marketing Tips” on a Weddings blog
2. do not have an author name / byline / bio . The article may be labelled as a guest blog, “paid promotion” or similar
3. have ‘money’ keyword-rich anchor text ( https://www.wordstream.com/anchor-text ) for the external links like 'best seo consultants london'
4. selectively 'nofollow' (https://support.google.com/webmasters/answer/96569?hl=en ) certain external links on the page for no apparent reason
Note that some link networks (created / used to sell links) block all web crawlers apart from Google so they can be nigh on impossible to detect (apart from if you are Google)!
So, to identify some of your competitors' paid / sponsored links:
1. Review / export a list of their backlinks using your backlink checker of choice. ahrefs.com/backlink-checker is free & suitable for site with links from up to 100 domains. Alternatives include https://neilpatel.com/backlinks/ . Paid alternatives exist too for domains with hundreds of backlinks
2. Sort the list by 'nofollow' tag and / or anchor text if the export allows, or you can check this manually in your browser with a plugin like https://addons.mozilla.org/en-GB/firefox/addon/nofollow-advanced/ .
3. Check the pages are still live with https://httpstatus.io/
4. Manually review the pages in question with Linkrr.com (a tool to open backlinks in bulk – you may need to adjust your browser settings -it works in Firefox) and revise your list as appropriate!
Google asks if you believe a site is engaged in buying or selling links that pass PageRank to tell them about it : https://www.google.com/webmasters/tools/paidlinks?pli=1 .
For guidance r.e. competitors' paid / sponsored links on Bing & Yahoo (both sites use Bing's index):