Do a long 'exposure' with the shutter closed and see what happens. That way you can tell the difference between fogging from 'flare' light that came through the lens vs light leaking in through the bellows or around the lensboard etc.
Yes. That's the ticket. Close off the system, isolate the variable in question, and test it under identical conditions.
"Hate is an adolescent term used to stop discussion with people you disagree with. You can do better than that."
—'blanksy', December 13, 2013