If you are still feeding your baby once in the middle of the night, and your baby is over 6wks old and weights more than 10 pounds you MIGHT notice that he doesn't really feel all that hungry at his 7am(morning) feed anymore. THAT'S WHEN YOU KNOW HE DOESN'T NEED THAT MIDDLE OF THE NIGHT FEED ANYMORE AND IT'S JUST BECOME A HABIT.
Maternity nurses for years have worked with a concept known as the CORE NIGHT. All it means is that once your baby sleeps for a large chunk through the night, you should never again give your baby a feed in that time even if he wakes; you should let him settle himself back to sleep; and if he can't settle himself you can pat him, use a dummy but not pick him up, as an absolute last resort some people recommend giving cooled boiled water, I found it created the same sleep association as giving milk, except that now your baby is waking up for the water (you are still not getting any sleep !).