The best way to learn is on your own chickens. A lot of people are confused about the crop mostly because it's impossible to feel unless it's got food in it. To feel what an empty crop feels like, feel the chest of a chicken before it eats in the morning. It will be flat.
To learn what a normal crop feels like, feed your chickens and when they're finished eating, pick one up and locate the full sack on the front of the chest wall just slightly on the right side. This is a good idea so you will be able to tell how a normal crop feels after a chicken eats and the normal size of it. This way, if a chicken acts sick at some point and you check the crop, you will know if it's normal size and consistency. A sick crop can feel totally different, but you wouldn't know unless you've felt a normal crop.
I commend you for wanting to learn these things before you find you have a chicken with a health issue.