Google Webmaster Tools will show you most of the links they have found.
If you are going to try to figure out which links caused a problem, you'll want to use ahrefs and majesticseo to compile a list of your backlinks, then either start sorting through them manually or use something like LinkResearchTools as a starting point.
Keep in mind that removing links will probably not return your previous rankings, since the links that once caused you to rank well may now be the ones causing the problem. Removing them may restore your ability to rank well again.
Also, not every ranking drop means that you were penalized. If you know you had built lots of spammy links, and your traffic dropped off at the same time Penguin was updated, then it is more likely a Penguin problem. Otherwise, there are hundreds of reasons that would need to be examined.
If you are not sure, consider hiring a reputable SEO consultant to help. It won't be cheap. Cheap is probably what got you in trouble in the first place.
In the comments of this article, a negative SEO perpetrator is outed. While Google's stance on whether or not negative SEO works seems pretty clear: possible but not likely, people like this continue to try, while perpetuating the FUD (fear uncertainty and doubt).
Is there something Google can do to deter this type of thing? While the outed spammer's petition to Google regarding spam is kind of ridiculous, his point that the belief in negative SEO is leading to more link spam being created is somewhat valid. But how could Google deter people from believing such things and acting based on those beliefs?
Keep in mind that removing links will probably not return your previous rankings, since the links that once caused you to rank well may now be the ones causing the problem. Removing them may restore your ability to rank well again.
Also, not every ranking drop means that you were penalized. If you know you had built lots of spammy links, and your traffic dropped off at the same time Penguin was updated, then it is more likely a Penguin problem. Otherwise, there are hundreds of reasons that would need to be examined. If you are not sure, consider hiring a reputable SEO consultant to help. It won't be cheap. Cheap is probably what got you in trouble in the first place.