Is the space an open return (no return ducts or partial return ducts)? Many years ago I could just shine a flashlight through the ceiling return grill; if I saw no sheet metal above the grill, the above-ceiling space was a plenum. If I saw sheet metal, returns were fully ducted back to the air handler.
Then they started using what I called a "directing duct" (a 90 directing the air flow toward the air handler); so much for the flashlight.
It became "up the ladder" and if there was even one return not fully ducted, the space was a plenum. I'm sure my use of "worst case" may have cost me some jobs, but it always kept me in the good graces of inspectors.
Regards!