Do not be a tattletale. I am in no way implying that you need to contact HR every time Betty prints her son’s permission slip on the company printer. It might not be right but perhaps Betty had to work late and didn’t have time to do it at home. It’s not your job to get other people in trouble, but it is your job to speak up for what is right.
If someone is being treated poorly or discriminated against based on race, gender, sexual preference/identity it is your responsibility to notify someone. If something feels wrong, it usually is. If something seems a little bit sketchy or shady it usually is. Don’t ignore these gut feelings.
Don’t get involved in other people’s battles, just notify the person who would handle it and then stay as far out of the mess as you can.