The usual forms of discrimination are race, sex, national origin and ancestry, color, religion, age, and even disability and medical condition. Each state may enact its own statutes prohibiting discrimination, but these cannot set standards less than the federal laws covering the same subject. California, for example, has liberal laws protecting workers from discrimination, and has no cap on damages that can be recovered. Federal law under Title VII limits damages depending on the size of the employer. California, for example, provides an array of protections for whistleblowers based on case law.
Many courts look to see if the activity is potentially criminal.
Other courts have looked for explicit legislative statements that the State has a fundamental interest in prohibiting the conduct. In California, the employee need only have a reasonable belief that the conduct is unlawful, and the complaint need only be made internally to management. Even a verbal complaint is sufficient. The difficult issue in these cases is showing that the termination was caused by the intent to retaliate months and sometimes years after the employee blew the whistle. Fortunately, California judges generally let juries decide this question for themselves as long as there is some reasonable proximity in time between the complaint and the firing or demotion.