1. A strong ammonia smell inside the box. This would indicate a leak in the evaporator section. If the unit has leaked for an extended amount of time the ammonia smell may no longer be present but you will probably hear a gurgling sound coming from the back of your refrigerator a few minutes after you turn it on.
2. If the controls are working and the cooling unit is heating up in the back, but there is no cooling taking place inside the box, this could be a sign of a blockage in the boiler section.
3. If your unit is not cooling but you don't smell ammonia inside the box, look for a yellow powder around or just above the burner located at the back of the unit. This would indicate a leak in the boiler section. Since this leak is outside the box you would rarely smell it inside.