I saw an episode of Ghost adventures recently where a ghost was "apparently" sitting on the end of a bed. They had a thermal capture machine show up blue to show up the coldness. What if you had some sort of machine that could measure the temperature of every molecule of air? This would possibly create a three dimensional image. I am aware that this sounds very much like Maxwell's Deamon thought experiment. Alternatively you could get a large sheet of paper, hold it above the ghost, and lower it. Although this would possibly anger the Ghost and make it move. The ghosts connection with air being cold seems to be consistent among ghost hunters.