Under the federal employment laws, workplace discrimination can be shown to have occurred by disparate treatment. This is where an individual is intentionally discriminated against because they are a member of a protected class, such as gender, racial or religious groups. It can also occur when some engages in a protected activity and suffers retaliation because of that activity, such as being fired after filing a complaint with Equal Employment Opportunity Commission (EEOC).
The process for analyzing if there was discrimination is a three-part test, first enunciated by the U.S. Supreme Court in the case McDonnell Douglas Corp. v. Green from 1973. The first part is that the employee must show a prima facie case of discrimination, such as a younger, less qualified employee is promoted and it is clear the more qualified applicant was not promoted because of their race, religion or other protected classification.
The second prong of the test allows the employer to put forward a legitimate, nondiscriminatory reason for the decision. With this, the employer does not have prove it was the genuine reason for their action, they only have to articulate it as a reason.
At the third step, the employee can then show that the reason the employer articulated is not credible and is merely pretext, because either it is factually untrue, or while true, it is not the actual reason for the decision.
This can be a difficult burden for an employee, and typically must be shown by extensive discovery during the litigation. Most larger employers are sophisticated enough that they seldom provide blatant direct evidence of discrimination.
There rarely is an email from a supervisor stating they are not promoting someone because of their national origin or their religion. And this is why you want to discuss your case early on with an employment law attorney, because they can advise you of the types of information necessary to build a successful discrimination case.
EEOC.gov, "EEO 101 – The Basic Theories of Employment Discrimination," Accessed April 7, 2015