A guy I work with (40+ yrs old) had a new girlfriend whom after a few weeks, he let stay at his house a few days. He figures out this isn't going to work out, but she won't leave. In my state, her saying she was his new roommate to the authorities meant he had to go through legal hoops to get her evicted. This took about 45 days: she ate his food, lived in his house etc and never paid 1 cent. He moved in with a friend because he couldn't get the authorities to remove her. She has many mental issues that he didn't see at first, she suffers from some serious delusions.
So no, you can't always know who's bat shit crazy. Look at all the serial killers who had neighbors, friends, etc that never knew prior to the arrest what loons they were.