When it comes to testing a fuel tank vent, there are a few different methods that can be used. The most common method is to use a manometer, which is essentially a pressure gauge. This will allow you to see if the pressure inside the tank is equal to the atmospheric pressure outside. If not, then there is likely an issue with the vent. Another method that can be used is to simply put your ear up to the vent and listen for any hissing or escaping air. If you hear anything, then again, there may be an issue with the vent. Ultimately, if you suspect that there may be an issue with the vent on your fuel tank, it's always best to err on the side of caution and have it checked out by a professional. They will be able to quickly and easily diagnose any problems and get your tank back up and running safely in no time!