At the most basic level, workers, employers, and clients all have the right to be treated fairly and with dignity and without discrimination due to race, sex, religion, origin, age, or disability. This applies to everyone in all situations. Yet these three groups do have some different responsibilities when it comes to discrimination due to their different positions within a workplace.
Employers, for instance, have the responsibility of creating and enforcing anti-discrimination policies. They are also the ones who receive complaints about discrimination from workers and clients and are charged with investigating those complaints, discovering the truth, and taking disciplinary measures as necessary. Further, employers must be extra careful that their own behavior does not lead to claims of discrimination, especially since they are the ones who hire, supervise, and sometimes fire workers.
Workers have the responsibility of acting without discrimination toward their employers, fellow workers, and clients. They also have the responsibility to report acts of discrimination when they experience or notice them. This is not necessarily easy, of course, but it is required.
Clients, too, have a responsibility to behave in a manner that respects a company's bosses and workers. They are also encouraged to report discrimination when it occurs although since they are not on the company's payroll, they are not required to do so.
As for the second part of the question, when someone within or associated with a company notices discrimination or a violation of rights and responsibilities, that person must first determine the proper channels of reporting (reviewing the anti-discrimination policy as necessary) and then report the incident in as much detail as possible. The employer, manager, or supervisor in charge of receiving and investigating such reports will then thoroughly investigate the claim, interviewing the parties involved as well as witnesses (if any) and trying to determine the truth of the matter. The investigator must look for solid evidence that proves the claim beyond a reasonable doubt. If this is found, the company will then take disciplinary action according to its policies. If proof cannot be attained, the company may issue a warning and continue to monitor the situation.