Recently I helped a friend who got a Google Manual Spam Penalty because he had some links in his blog considered paid links. Those links was not really paid link but the whole post was a paid post so… they can be considered paid links at the end.
That is not prohibited by Google, but the search engine requires those links to be marked as “no follow”.
With my friend we agreed that was better to mark all post outgoing links as “no follow” instead of search for them on hundreds of posts (and actually not knowing which links were offending Google).
So I modified an old plugin, Outbound Links, to do that. It can be useful to you as well.
The original version was able to track on Google Analytic clicks on external link, but since the Analytics code is changed over those two years it was no more working. I’ll try to read such functionality!