If you're lucky enough to have kept a record of the linking domains, you could search to see if Google has taken some sites out of their index by doing a "site:" search, ie:
site:domain-that-links-to-me.com
and if you get no results, I believe that's because Google has de-indexed them.
If you pay for a lot of content sharing type links, they are likely to be on sites that Google will get round to eviscerating eventually.
What percentage of total links have gone?
