If the man is too nice to the young female waitress it's probably not a good sign either.
As is being too nice to the young male waiter. 
I think it relates in general to how they treat people who do things for them. Are they polite, generous, civil, understanding, or are they rude, tight, aggressive, dismissive?
The waitress specific situation may not be a great indicator because some people are shy, but we don't often see possible partners dealing with people that work for them.
Alternatives may be to ask them to teach you something that requires patience. Or work on a common project with them leading.