Patrick Danner of the Express News is covering this story involving a Medal of Honor winner who has brought suit against his former employer, alleging the employer defamed him and interfered with his ability to seek gainful employment.
As the article details, proving that an employer is black-balling an employee by giving out false and defamatory information about you can be extremely difficult. Before you call a lawyer, however, keep in mind that it is perfectly legal for an employer to say not nice things about you to a prospective employer . . . as long as the statements are fact-based and TRUE.
There are third-party providers who can attempt to check what type of references you are getting (a lawyer cannot do this for you). These usually run between $100-$200 and can give you some piece of mind if you are concerned what type of reference you might get from your last boss.
If you have a serious indication that your past employer is making false, defamatory statements about you that are harming your ability to get hired, then you definitely should consult with an employment lawyer. There may be steps short of litigation that will resolve the issue.