Q: How can one find out if he has been affected by Negative SEO?
A: Negative SEO is a big concern for webmasters. When a competitor complaints about your website, this could result in affecting your ranking and getting you a manual action from Google.
If you have been affected by negative SEO, you must have done something to violate the Google Terms of Service. This could be an on page violation like keyword stuffing or an off page violation like participation in a link scheme.
It is however true that anybody engaged in SEO is violating Google’s rule. Google has thoroughly written the rules that there are quite good chances of almost anything to fall under the violation category.
Assuming that you haven’t violated any of the Google’s guidelines but are still impacted by negative SEO. The reason could be your competitor. He might have noticed that you are ranking high across searches and could have logged into webmaster tools to issue a complaint.
A complaint or two are usually not enough to get you manually penalized by Google. However, if your competitor knows what you are doing or has contacts inside Google, it is enough to catch you. The first step Google would take is to alert you via webmaster tools about the manual penalty and will guide you on how to fix the problem.
Therefore, it is important that you are subscribed to Google webmaster tools and do check for alerts at regular interval.