A crop feeling like a balloon is the first reason to suspect a crop disorder.
The next step is to check the crop first thing in the morning to see if it's emptied overnight. A normal crop will be flat and empty. A sick crop will have contents that refuse to go down, sort of like a toilet that won't empty when you flush.
If you find the crop still full before your hen eats or drinks anything, and it is very important not to feed her or let her drink until you check her, try to determine if the contents feel spongy or hard and lumpy. That will determine which crop disorder you will need to treat.