One thing it is hard to do as investigators is to remain objective, for obvious reasons. We research the paranormal because we have an expressed interest in capturing activity, the greater the activity the better. This being the case, it is easy to see something that isn't there. Shadows, reflections, polarizations, all of these things would be meaningless to the average observer, but to a ghost hunter it may be all we need to justify our hobby or meet whatever need we are trying to fullfill.
I find one of the easiest ways to overcome this natural inconvenience is to try to look at things as a disbeliever or a skeptic. If the average Joe on the streets couldn't see what you see after pointing out it's location, there is a good chance that you are seeing something you want to see. Ask yourself these questions when attempting to rule out your imagination:
What would the skeptic see in that photo?
When you showed it to a few different people, did you have to explain it to them beyond pointing out it's location?
If a skeptic can see it, and you don't have to explain to most people what it is that you are seeing, then there is a better chance that you have found something. Does this mean that the figure of a man in your hallway is definately paranormal? Of course not, this is just a small piece of the puzzle when evaluating your findings.
Some reading that might help you better understand this concept is here:
Occam's RazorAs long as we remain objective, we will go much further in our research.