Matt Cutts Google's head of Webspam Team, has released a video explaining how a webmaster can locate which links pointing to their website is “unnatural".Google has been sending unnatural link warnings to Webmasters for about a year now and Webmasters should act swiftly when they receive this email.Here's an example:-

Dear site owner or webmaster of ….We’ve detected that some of your site’s pages may be using techniques that are outside Google’s Webmaster Guidelines.Specifically, look for possibly artificial or unnatural links pointing to your site that could be intended to manipulate PageRank. Examples of unnatural linking could include buying links to pass PageRank or participating in link schemes.We encourage you to make changes to your site so that it meets our quality guidelines. Once you’ve made these changes, please submit your site for reconsideration in Google’s search results.If you find unnatural links to your site that you are unable to control or remove, please provide the details in your reconsideration request.If you have any questions about how to resolve this issue, please see our Webmaster Help Forum for support.Sincerely,Google Search Quality Team

But what if you really do not know which links pointing to your site are unnatural?

Google’s Matt Cutts gives you two ways to discover these links.(1) You can sort your links to your site within Webmaster Tools by most recent links.(2) Google is now often sending examples of unnatural links within the unnatural link warning emails to webmasters. They may list a few examples of the types of links Google finds to be unnatural; this way, a webmaster has something to work off of to start their process to remove the unnatural links.

Video from Matt Cutts on Unnatural Links in Google