An empty crop feels as though there is no crop. You should feel only the chest wall. I there is something squishy in the crop in the morning before a chicken gets down off the roost, that is a signal there is an issue with the crop. Checking the poops under the roost where that chicken spent the night will give you further information about what the crop issue might involve.
No poops at all might indicate an impacted crop, as would scat, hard , dry poops. A liquid puddle of poop laced with white mucous and bits of green matter might indicate sour crop.
Behavior will tell you even more information about how the chicken is feeling. A stationary, mute chicken facing the wall or corner with tail held low and flat will tell you the chicken is sick or in pain.