How to find bad links:
Best way is to maintain a spreadsheet of what links you made and what anchor text you have used so far to make links. Now, If you have done this already then open each link. Now make use of seomoz bar to check the Domain Authority of that domain sending link to you. Anything less than 40 DA can not be called as high quality link (though it might still be passing some link juice).
Further, you should check whether it is a link in comment or from directory submissions. After recent Google updates both these links are considered spam in most cases and should be removed.
You should also analyse your rankings and Google Analytics data. If the organic traffic dropped after the dates when you made link and if there is any drop in the rankings after you made those links then these can be seen as the signals showing that those links might be bad links.
Few more obvious factors are If the link is coming from a site which is filled with spam, a paid link, reciprocal link are kind of links which are not good for the health of your site!
Removing Bad links:
Checkout each of the site and login to your account and either remove your account or just remove the link. Or else you can look for the author or contact person on that site and send q request letter with your login details requesting them to remove the link.