Question: how do you know if you put too much gas in your car?
Answer: Coo Coo Ka Choo. If you top off the gas to the very top when you fill up you might have a bad canister. If you stop filling as soon as the pump valve clicks off and don't add more gas after that, then the canister is OK. Usually a CEL will go on if the canister is bad as part of the car's emission control system.