You can test it with compressed air and glass cleaner. Spray the cleaner around the inside of the gasket(inside the car) and hit the out side with compressed air from about 6 inches away. Any bubbles inside and it's a leak. That area can and should be sealed but if the pinchweld is good and the gasket did not get cut and the right gasket is used, it should not leak.
Folks screw up by using a gasket made for chrome trim and if the trim is not installed it will leak. The trim expands the gasket.