The answer is going to vary, and if she’s always been that way, it simply could be who she is. I have several books that cover the potential signs, and the major issue is- there isn’t a big difference between the signs of being possessed, cursed or simply a bad person, unless it’s been a sudden change. I found this post in the search bar that list many of the typical things for you to glance over.
Something I’ve noted though, is a few of the books say to take action if you even simply suspect it- because you thinking there might be a curse can certainly generate one, so if you take actions against it regardless of being sure, you can at least have the piece of mind knowing you have stopped it, if it is a curse.