The easiest single source of finding the legal definition of "cause" is in Black's Law Dictionary. You can also try looking on line on search engines using "cause" and "employment contract". There are probably articles in AVVO, FindLaw or NOLO on the subject.
Basically, cause means that a substantial reason existed for termination. Examples, chronic attendance or performance problems that were not corrected after warning, insubordination, failure to maintain confidential information, stealing, etc.
One way to test the employer's stated reason is to file an unemployment compensation claim. Under law, the employer has the obligation to prove that you committed misconduct or gross misconduct to disqualify you from benefits on a partial or complete basis. The standard is slightly higher than cause. However, if you win, it is unlikely that the employer will come after you.
The other alternative is to consult an employment attorney. Acting proactively and addressing the issues before they get out of hand may very well be the best course of action. Litigation is expensive if you have to defend against a suit and you have potential bad references lurking out there too.