There are two ways to check whether a door seal is intact:
Put a switched-on torch in the refrigerator and close the refrigerator door. Switch off the light in the room briefly. If the refrigerator door is leaking, you can see the light from the torch from the outside.
If your unit has a glass door, use a piece of paper to check for gaps in the seal. Close the door with a piece of paper in between and pull it out. There should be tension when you pull the paper out. If this is not the case, a defective door seal could be the problem. Repeat the test along the entire seal.