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The only way to determine what is being said about you is to have a friend or private investigator call and pose as an employer requesting a reference check. There is nothing illegal about doing this, and it should give you a general indication of what your ex-employer is saying.
Although it may not be too hard for you to figure out what your former employer is saying about you, your options are unfortunately quite limited if it's bad. This is because it's not illegal for an employer to state unflattering opinions about your performance as an employee, if that is indeed what is occurring. In other words, if asked, a former employer could say in response to a reference request "I'd never hire Bob--he was a horrible employee" and no legal cause of action would arise. This is a statement of opinion, not fact, and thus could never give rise to a claim for defamation (defamation arises only from false statements of fact--that is, those statements which are objectively true or false). An employer could even say "Bob was fired for stealing" even if nothing was ever stolen, provided the employer had a good faith belief something was.
This obviously leaves employees are are receiving less-than-flattering references in a difficult situation. So, what are the options? Well, you could hire an attorney to write a "cease and desist" letter demanding that derogatory statements immediately stop. But for the reasons noted above, the letter would essentially be an empty threat. You'd have to hope you simply scare your former employer into changing their ways. It's possible, but you'll be spending money for a service that may very well not yield a result.
The other option is to list as your reference a co-worker or manager from your old company whom you are confident will say something positive about you. Sure, it's possible prospective employers might directly call your former employer's main office and get in touch with someone else, but they may simply call the person you list as the point of contact. There is nothing illegal or even misleading about that. The prospective employer is still getting their reference, just from someone who will provide a more accurate explanation of your value as an employee.
The last option is to simply address the issue head on. If there's a reason why your old boss is saying bad things, you can simply explain that things were left on less than amicable terms and to expect that will be reflected in their response to a reference request. If you provide several other people as references who will vouch for your ability and integrity, many employers will look past one obviously disgruntled former employer.
These are really the best options here. I realize they are not ideal, but until the law is changed to provide employers with less protection to give biased and unfair references, there is little more an employee in your circumstance can do.
I hope that you find this information helpful. Please do not hesitate to let me know if you have any questions or concerns regarding the above and I will be more than happy to assist you further.
If you do not require any further assistance, please be so kind as to provide a positive rating of my service so that I may receive credit for assisting you. Very best wishes moving forward.