Just feel it. In the morning, before he eats, it should be pretty flat to his throat. If you feel it after he eats, you can feel a ball of food there. And just in case you don't know (don't want to assume) it's located on the left side if you are facing him. It's pretty evident when they have eaten. It should empty completely every night. If you check now, if there's anything in there, it should be fairly soft, not rock solid.