You can disprove your employer's stated reasons for your termination using the same kind of evidence used to show pretext in a discrimination case. For example, you can show that the employer's excuse is factually untrue, that it was insufficient to have actually caused your discharge, that it is simply unworthy of credence, or so riddled with errors that your employer could not realistically have relied on its stated reason. Remember, it is always going to be up to you, the employee, to prove unlawful motivation. While difficult, it is not impossible. Circumstantial evidence can be powerful in proving your case.