If you receive an anonymous complaint about a hostile workplace, launch an investigation right away.
That way, if an employee later sues, you can easily compare what he said to the investigator with what he remembers now.
Recent case: Elfego Rodriguez worked for the Burbank Police Department and sued over alleged racial slurs.
But earlier someone had anonymously complained about racial slurs, prompting an independent investigation. Rodriguez was one of the employees interviewed. He told the investigator that he had heard a slur or two during his first year with the department, but none since.
The court held him to his earlier account and dismissed the claim. (Rodriguez v. Burbank Police Department, No. B227414, Court of Appeal of California, 2nd Appellate District 2012)