A simple paper test can reveal a weak section in a refrigerator door gasket. It does not measure the seal scientifically, but it helps locate areas where cold air may be escaping.
Clean the gasket first
Wipe the rubber seal and the mating surface with warm water and mild detergent. Remove crumbs that can hold the door slightly open.
Test several points
Close the door on a thin sheet of paper at the top, sides, and bottom. Pull gently. Resistance should feel reasonably similar around the perimeter.
Check alignment
Look for a folded corner, a gap, or a door that sits low. Do not heat or reshape the gasket aggressively; follow the appliance instructions.
Know when service is needed
If the gasket is torn, the cabinet is not level, or frost keeps forming, use the model information for the correct replacement or request appliance service.