Turn your car on, keep the hood up and keep the cap removed. When you start your car, you should see some bubbles initially, that is fine, let the car heat up to normal temperature. When it does, check to see if you see any bubbles (white foam like) starting to appear. If you do, you have a gasket issue, most likely a leak that is allowing gases to enter the radiator. This will build excessive pressure, boil water, and cause a heat up at some point in time. If you do not see these bubbles, you shouldn't worry about your gasket too much.
Note: you do not have to wait and see if the bubbles appear when the temp is normal running, you could see them before (but always ignore the ones you see during ignition).
Hope this helps.
Rao.