systemhood is just a way of modeling a broad set of experiences, and all the medical diagnoses related to it are models for describing patterns psychiatrists observe in some patients

if a model in which you are multiple people proves to be more useful and make more accurate predictions than any model in which you are one person, then for practical purposes you are a system

if no such model is as useful or accurate as a model in which you are a single person, then for practical purposes you are not a system. in other words, you can approach it scientifically

you may also find that both are useful theories in different contexts, or that in some contexts neither is useful... when I was trying to figure out whether I was a system (and eventually decided that I was not) I found that the most harmful thing was trying to fit every experience into one cohesive model of the self, and to attach my identity to that model