If you are wondering if negative SEO has happened to your site, there are two simple ways to determine this:
If you have a notice in Google Webmaster Tools for either malware notification or unnatural link building, there is a good chance negative SEO has taken place (assuming you didn’t do any unnatural SEO to your own site).
If your analytics data is reporting a huge drop in search traffic, there is a good chance some negative SEO happened (assuming you didn’t do anything to break Google’s webmaster guidelines).
Once you’ve determined something has happened, here’s how you go about fixing it: http://www.quicksprout.com/2014/03/17/how-to-recover-from-negative-seo/
Via Antonino Militello