I'm not sure that you can know for sure but, I decided the person who bullied me likely was because so many of their behaviours aligned: Bigging themselves up whilst putting everyone else down, gaslighting, outright lieing and (falsely) accusing people of things where they became the victim. Roping other people in to their (false) naratives. (Flying monkeys.)
I went from being terrified at the time. Wondering if I was the crazy one and maybe that I had even slept walked to do some of the things they accused me of! Later on though, I realised it was all them. Then, I became puzzled. Why would someone outright lie over and over again, trying to get someone else into trouble? I wondered whether they believed their own lies. I wondered if they were psychotic. Then, I stumbled upon the definition of narcissism and, so many of their behaviours were there. I'll never know for sure of course but, it gave me a huge feeling of validation. That finally, there was some sort of reason for it all.
As for whether someone is toxic or bad, that's tricky. Are they behaving in away that hurts you? I think that's the main thing to acknowledge and to try to get away from. It maybe isn't fair to call many people outright toxic or bad but, I think it's important to realise what effect they be having on us and to protect ourselves accordingly.