Get to know your singing tools. The more familiar you are with how everything is supposed to move and feel.
Find your chest line. This is the place where your lungs expand the largest. Place your hands about halfway down your torso, towards the bottom of your sternum. Take a deep breath, and move your hand till you find the point of maximum extension of the ribs.
The bottom of your lungs is right below your sternum, where your rib cages meet. This is also the housing for your diaphragm. The reason your stomach may pooch out when you breathe deeply is because your diaphragm is pushing down on everything below your rib cage, not because your lungs are in your stomach.