If the server has a look of astonishment and doesn't physically react at all to my return of serve. and I have some doubt, I usually give him a break and go to a second serve. Sometimes sun glare or shadows or crazy spin makes it hard to see the serve landing spot. The server can sometimes see long faults better than the returner. It is reasonable, in a physically demanding unofficial match or if the server is playing with a minor injury, that he doesn't want use energy to chase down a return of serve on an truly "out" serve.
According to the Code, servers are supposed to play every serve like it is "in" so this is technically not the correct way to play. I would be less likely to give a break like that in an official tourney.
Of course, if you see the serve as 100% clearly in, stick with it.
Last edited by rufus_smith; 01-08-2013 at 08:36 AM.