I think it's good that the gate agents try policing the boarding groups and turning people away before they get to the BP scanner makes it easier. In case the "group 1 only" reminder is generic to everyone approaching the gate, there is no reason to be offended. If they say it only to you, then it's more embarrassing for the agent than it is for you.
Just looking at what kind of people sit in the front of the plane I'd say there's no way of telling from the looks who is going to sit there. However the gate agents might have a more developed instinct as they process hundreds of passengers each day.