Let me give you mine:
1) She has a goal of her own, not somebody else's goal.
2) And some resources with which to pursue it, such as personal skills, virtues, or tools.
3) So then she runs around doing proactive shit to the struggling plot while the author screams, "No! NO! It wasn't supposed to happen like that!"
4) Then she flips off the author and does whatever the hell she wants.
5) Also, if there are guys interested in her, she does not fall in love or bed with one who is obviously an asshole. (Secret assholes are different. But they will Get Theirs when she finds out their true nature.)
More simply: if she's doing things, she is a hera. If things are being done to her, she is a heroine.