How do you tell if your radiator cap is faulty?

take the cap off and check this look under the cap and see if the rubber seal is broken or it looks worn but if in doubt replace it a mechanic has tool for connecting to radiator to check cap pressure. but if you think radiator cap is the problem, just buy a new one they cost under 10 bucks. if that is the problem, it is sure cheaper than going to a mechanic