How serious are you about finding out? If you want to invest time into it, round robin applications. For example, if you need 3 references per application, ge

How serious are you about finding out? If you want to invest time into it, round robin applications. For example, if you need 3 references per application, get 4 then cycle them in sets of 3 each time. Or find an application that requires only 2 references and split sets. Eventually you will have enough data to know who the bad reference came from. This is time consuming, but will net you your answer. The truth is, your options are drop all the references you used, even the ones you think are solid, or find out who the lame duck is and delete them. Legally, no one can say anything because it opens them up to a lawsuit, so stop chasing that avenue unless you know them personally. Good luck!