Hello NarrowPath.
I was researching demons on google as it came to mind one again and I found this topic - I think it's interesting what you are asking. I often thought the same. I am also a Christian, but for some reason God lets me see demons in people sometimes -- just a quick note at this point, i am replying to NarrowPath just now, and if you don't believe me, please don't post flames about what I am saying here -- I have seen a "demon" that looks something like an "alien" outwith another person though, and it was staring at me through the window.
The reason for me seeing all that demonic stuff when I did though, i believe, is because i actively opened a portal for it. To start with, i was a Christian - i was open and aware of the spiritual side of things in life. But then, i made an active choice to get into witchcraft. During that time i saw a lot of untoward things, and had scary experiences. But never did I get attacked by any demon.
I have seen one demon in different people - the most obvious point always seen through the eyes. A saw an old woman, but her whole eyeballs were completely black, my friends couldnt see it though, as i was trying to point it out without totally freaking out!!
I have always thought that you cannot be possessed as a Christian, because you already have the Holy Spirit in you. You're "possessed" already by the Holy Spirit!!
I DO believe, though, you can make a choice to go against everything God says and go the opposite way and end up getting possessed though. Spiritual openings are spiritual openings, thats why i will never ever play tarrot or ouija boards.
I am digressing a bit now. But i hope you get back to me about this and let me know what you think.

Thanks.