References like past employers have a friend call for a reference to see what they say. If you have a friend in HR have them call and try to drag negatives out of them. Most companies have a verify employment only policy, because negative references are almost always considered libel. Because this (California) is a right to work state it means employers have no need for cause to fire you. Even telling another employer you fired them can be considered libel since there was no requirement for cause for the dismissal. All companies I have worked for have a only verify employment reference policy because of this. After all has anyone ever called for a reference and the company simply admitted, a supervisor didn't like X so we fired them.
If you are getting negative reviews get an employment law attorney immediately. They will work for a cut of the action, and if all you need is a cease and desist letter it will be a couple hundred bucks well spent. It will either get the negative reference to go away and get you the next job, or give you attorney enough information to file suit.
No matter what always have a third party call your references, pretend to be a perspective employer and check out what they are saying.
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