good question
Crop problems - indicators
If the bird stops eating
If the crop becomes rock hard and doesn't go down by morning
If the crop is distended
If the bird is dribbling fluid from its mouth or it runs out of the mouth when you pick the bird up
If the mouth area smells really bad
If the bird start to vomit
If the bird stops eating
If the droppings turn green, it can idicate crop problems and your seeing undiluted bile
Many many different indicators as to a bird that may or may not have a crop problem
The best advice I can give you is... go and spend time with your birds, watch them, pick them up, feel them, feel their crops, feel their combs, feel their legs and feet, look into their eyes, look at the feathering, look at the base of the feathers, look at the vent area
Watch them, study them, then when one displays characteristics that are not what you feel is normal... act on your gut feelings and do something about it .. ask questions if your not sure.. giving as much detail about the BIRD not the country side or coop or how you found the bird, concentrate on discussing the bird and the symptoms