Undercover volunteers who pose as prospective buyers or renters and visit or telephone real estate sales or rental offices to determine whether customers are being treated equally, are called

fair housing testers

Explanation:

Fair housing testers are the volunteers who pretend they want to buy an apartment. They do that to see how they are being treated, if belonging to a specific ethnic group. This way, discrimination is identified if different information is given to two individuals belonging to a different ethnic group.