how to define bad backlink

My ecommerce site has been notify as violated google policy,  how do I do to identify those links as bad links for removal?

Any details on what they complain about?

it did not mention  just unnatural links

Use a link diagnosis tool(s) and let it do the work. There are services on BHW if you don't want to buy a subscription yourself.

If you give me the URL (by PM) I can run it through my tools and give you a CSV file of what could be the potential danger links. 

If you want to do it manually, look at the types of links. Forum profiles, badly spun/duplicate content contextual links, mass blog comments with the same anchor could be classed as a bad link but these new tools that are out nowadays look at far more factors. Google usually gives you a few examples of bad links in WMT, unless it hasn't been manually reviewed (yet) then you should be OK to get rid of some links.

JudderMan  PM you already