Well, is she driving you crazy? Then she probably is.
Can she take care of herself? Can she keep a job, pay her rent, etc? If not, then she probably is.
Have you lost your sexual attraction to her, but feel the need to stay with her because you feel sorry for her? Then she probably is.
If your girlfriend is insane, her family needs to know about it. Of course you can't just abandon her. But you are also in no position to help her. The best way for her family to find out and for her to get help is for you to commit her to a mental institution.
This is life lesson that I learned first hand, and it took me over a year. Hopefully, you'll take less time to do what's necessary.