When you test something's validity, you see if it holds true. Are these really the best chocolate chip cookies in the world? You might enjoy testing the validity of this statement.
The noun validity means genuine or authentic, but it also has a legal meaning: having legal force. Your family asked the judge to determine the validity of your grandfather's will because they weren't sure he was legally allowed to leave all his worldly goods to "the little green men from Mars." Related words include the adjective "valid" and its antonym "invalid," as well as the verb "validate."