I've seen such things before. [It's a sore subject; the leader of the Seekers, her group - her family, was corrupted and forced others in the Seekers to be possessed. As a result, she had to kill her apprentice. among other incidents, of course.]
It might not always be noticeable. If you know the one possessed, it is one thing. But those who are brought here, alone and without that support... [there would be ways to tell; those with magic should be able to figure it out, eventually, but...]