You see, the great secret, Eliza, is not a question of good manners or bad manners, or any particular sort of manners, but having the same manner for all human souls. The question is not whether I treat you rudely, but whether you’ve ever heard me treat anyone else better. — My Fair Lady

You can tell a lot about how someone treats people who cannot benefit them.  Do their manners change when they’re dealing with a person behind a cash register?  Are they nice to the car salesman, but treat the mechanic like crap?

If they treat the lowest of the low as well as they treat the highest of the high, then they are truly well-mannered.