A grouchy person treated with consideration and caring often responds with an apology, and takes hold of themselves to behave more civilly. I think that courtesy and right behavior often reflect good character. Of course, courteous cads and villains abound, so this is not an absolute indicator. Certainly what one is, is of far greater importance than what one appears to be. Most of them are out of our control.
Consistent choices become habit.
And habit is the building-block of character. Thus, our knee-jerk response to being cut off by another driver is our choice, not inevitable. Hold the door for someone. Do your personal hygiene in private. Close your mouth when you chew.