You can not tell without docs. If you find the crayon codes on the bulkhead, it would be nice. There are things to look for but there is nothing to say for sure if the engine and paperwork are missing. Lots of things to check to lead you to believe that it "could be" or "could not" be but nothing there to "prove" it.
Tom, you could exchange the M20 for an M21 or vice versa optionaly at no cost on any of the higher performance engine cars that would have come with a Muncie. The M22 was an extra cost option.