When is discrimination at the workplace unlawful?

In the US: When you have legal counsel powerful enough to expose it as so (while simultaneously capable of protecting your constitutional rights and livelihood.)

I think the thing is proving it, which may be difficult.

When it goes against Federal, state or local law or company policy, it is unlawful. Proving it is a secondary issue.

At my work my bosses were sued for age discrimination, violations of the Americans With Disabilities Act, and gender discrimination, to name a few.
Sometimes the suits had merit, but most of the time it was because someone didn’t get their way.

When you get caught!

Why do you ask?
Is this for research purposes?