I have a friend who valet parks at an up-scale hotel, and over the years he's become pretty good at sizing up whether someone isn't aware that a tip is expected. So he's come up with a little code between his fellow employees. While the newb who doesn't know he's supposed to tip is waiting for his car, my friend loudly announces to a fellow employee as he hands him the keys to fetch some other customer's car "hey, he's already tipped, so he's good to go...". Without fail, the newb promptly pulls a $5 out of his pocket a few moments later...
It's just like the loose change in the street musician's instrument case. They always start with a little of their own money in there because it helps people understand what's expected.