I am questioning whether or not a previous employer is sabotaging me and saying untrue things, how do i prove?

I was at an interview and it went really well. The three people I interviewed with pretty much told me i had the job, but they had to go through the process of interviewing two more candidates. I was working for a company that was an extreme hostile work environment that i had to get myself out of. I accidentally put the contact number for the supervisor that was making my life hell on my application from the previous job. I think they are part of me not getting hired. Can I legally ask why I wasn't considered after being told I was going to get hired? I want to catch the old supervisor in the act of breaking the law of sabotaging my name. How do I prove it?
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What a previous employer can say about a former employee is regulated by state law. The majority of states will allow only length or service, title/job duties, and some salary indications (for some states it is a range, others can get exact figures).

There are a few states, Illinois being one, that allow just about any information to be passed. For example in Illinois a previous employer can say what they want as long as the employer has a belief that it is true.

Employees can fight this by never, ever giving the phone number of the direct supervisor. Give the number of the HR dept. HR depts. usually will follow the law or the company policy. Companies who are in multiple states usually will opt for the most retrictive policy on information.

Even if you do all that, there is always the potential that someone will tell someone will tell a friend of someone at the potential company. So, the employee needs to ready IF ASKED to say 'my previous supervisor and I had a personality conflict so I do not expect an accurate recommendation form him, instead speak with this other person who knows my work'. Make sure you have one or two people who know your work and will provide you with good references.

You can legally ask why a company did not fire you, but they will probably give you a vague line. Why? Because if they admit that they relied on the reference from the former employer you can get this potential employer involved in a lawsuit. That is how you would need to proceed if you want to sue the old supervisor for defamation. You need to put on the witness stand an employer who will say under oath that they did not hire you totally due to the poor reference. No employer wants to step in that situation.

So find some other people who know your work that you can use as references, and do not put your former supervisors contact information anywhere on an application. Good luck.
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you can ask, they dont have to respond. It is illegal for former employer to bad mouth you but proving it is very hard to do. They can answer if you worked there time of employment and if you are rehireable or not. ALOT can be told by the tone. I have checked refrences before and could tell by the tone on the other end of the phone they didnt like the sound of the name i was calling about.
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You could try getting a friend with a manager-sounding voice to phone up pretending to be a would-be employer. Or perhaps it might be better to get your solicitor to do it, as they would be a more credible witness, but it would probably be against their professional code of conduct to lie.

Remember that when applying for a job, you can quote previous employers as referees, but say that the details of the most recent employment are "available on request", so they do not ask for it until they are about to give you the job, and phoning your most recent employer is the very last thing they have to do before they make a decision.