How do you know when you're being too much of an asshole?

I like to be playful with girls, but occasionally I might say something without thinking that could be considered too assholish. In fact, just last night one of my flatmates (male) remarked that he was "pleasantly surprised at the fact that [I was] being nice to a girl for once; [I am] usually a twat." This got me thinking, what's the best way to stop yourself from being too much of an asshole?