I want to be cautious and know all the red flags. I don't want to get burned again in a relationship.

But I also want to be careful not to mislabel or misjudge someone.

This PC article has been helpful in this regard:

"Borderlines...more appropriately named as “emotional dysregulation disorder” instead (Houben, 2016). While narcissists can also be emotionally explosive in their rage, due to their need to have a “false mask“ or public persona, they have more impulse control, can fly under the radar, control their behavior more easily if there is a witness present or if they need to engage in impression management."

The Differences Between Abusers with Narcissistic Personality Disorder vs. Borderline Personality Disorder

I've noticed this difference also. People who are simply struggling with mental illness manifest their behavior in public. But a narc abuser only shows his or her real-self to you.