There were alot of links added that link back to the site, we never built links but have quite a few of them and the site rankings went down about 1 year ago after the google search engine updates to the algorithym How will I know what links are bad and which ones should I disavow them Just added the site to google webmaster but there is no data there at all
Here is a reference that might help, http://www.linkremovalguide.com/tips-and-resources/identify-bad-backlinks/ I'm not sure if one of those tool will help you identify the links that are linking to your site a year ago but you can see and give it a try. On the other hand, if you are unable to identify the bad backlinks then I suggest that you start covering/replacing it with good backlinks.
There are three popular services to help you find backlinks. https://majestic.com/ https://ahrefs.com https://moz.com/ I personally like Majestic because it has a very decent backlink database and is also the cheapest among the three. You can also easily see which ones are bad links based on the trust flow of the back link.
Only such that will show you factors that usually are connected with bad backlinks. Its always good to know what you are looking for so you can be sure while identifying. Some usual "suspects": - irrelevant spammy links - site-wide links - deindexed websites - websites with a very low Trust Flow - footer links - comment spam Those are just to name a few. Making frequent link audits might not be the easiest and quickest job in the world but it is definitely of high importance, moreso with Negative SEO still being possible.
You can also use Ahrefs, but be sure that the Google Webmaster Tools are almost everytime much more accurate then other those SEO tools available online!