Guy i knew was macking on a girl friend of mine. The girls all knew his reputation of being hung and pretty amazing at sex (as amazing as teenage-high-school sex can get). The guys all knew he bragged about being abusive and disrespectful of the girls he'd bang.
I tried to stop her... told her she deserved better... she got mad and told me to stay out of her business...
A week later she came crying to me and told me she should have listened to me.
That's really the only time where I "knew" the guy was bad news.