How do you know the person your involved with us a narcissist?

I don’t know if he is a narcissist or just a very broken ken damaged person. I know he was married to a narcissist who cheated on him. He was in prison for 11 years and his family never visited him. The way he treats me is not so great. I just can’t tell if he is one.
How do you know the person your involved with us a narcissist?
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