I am always asses my children as I was allowed as a child to hold extra weight and I fight with it all the time to try and fend it off. Our eating habits never improve with age so a real healthy eating habit young has a great payback later in life.
My oldest is really awesome at monitoring himself. My youngest is just too picky though never snakes if full. He just stops eating even part way through a meal. My grand father was the same way be eating then suddenly stop and pour a tea and say I ma full. leftovers he put away and ate the next meal as he did not like to waste.
I take a active part in what my children weight. I do not want them to be over weight but I also do not want them to be too underweight. I take in account their size their activity level and how much they eat or snack. I do not want to send my kids out in the world without fully realizing that their health is something they need to take a active part in.