Hi,
You have to contact site admin of those sites and request to remove your links from those sites. If it is not possible for you then you can hire any link removal companies.
Also you can add all bad backlinks to Google Disavow.
It depends on the sites where you built up backlinks earlier. If you posted on blog sites, you can just contact the webmaster and ask to remove the links. If you posted on forums, you can just log in and delete the links from the posts or signatures.
May be that you could try preferring "Disavow tool". Looking for bad links in Webmaster Tools may take some time to acquire the result instead, you can opt for the tool mentioned before.
Matt Cutts have now put on the information that websites can use the disavow tool even if they haven't received any penalization message. This would not bring in a negative impact to the website like previous days.