How do I check to see what is wrong

If it is only the one door, check the door switch.
Look for a push or slide type switch around the door frame. It will be near the door latch in most cases as its off position is actuated by the foor closing and pushing it to the off position. Sometimes they get loose and fail to not disengage, especially the slide type switch. I don't have a schematic for the switch location, but it will be the only thing besides the door latch that moves back and forth in the door frame.
If you remove it with the wires disconnected and the light stays off, it needs replacement.