A good way for sure to tell if she's cluster b are:
If other people say she's crazy.
Beneath every woman whose behavior seems unexplainable there are a series of overlapping issues. If you knew the issues, the situation would appear perfectly clear.
Whatever these issues are, they are cascading and thus you have to resolve each one, often in reverse order.
For example, she wasn't eating right, went on prescription meds, then got kicked out of her house, then started dating the wrong guy, then started stripping. So you get her to stop stripping, get her to stop dating the wrong guy, get her back close to her family, get her off meds, and get her to eat right and you have solved the problem. This requires getting to know them and truly wanting what is best for them. Most people simply create another problem and tack it on the end.
A simpler example is you told your girlfriend something that hurt her feelings (without her telling you), then she said something to hurt your feelings (which seemed to come out of nowhere), and now you're not talking.