Hi Spockey CASEY76 replied to someone who queried this as follows:
All hybrid laying hens are thin and light. You can feel their breast bone easily. A lot of people (including vets) are often surprised at how lean layers are - especially in comparison to heavy pure breeds, or meat birds. A hybrid layer may weigh only 1.5 - 2kg, however every hen differs, and the best thing you can do is handle them a lot, and get to know how each hen feels so you can monitor to see if they are losing weight.
Second of all, to see if she has sour crop opposed to a crop impaction, you will have to pick her up and have a good (but gentle) feel of her crop. Sour crop usually feels "soupy," and very often the hen will have very bad breath. This is because the food in her crop is fermenting. To help relieve the hen, hold her so that her tail is pointing towards your elbow, and support her underneath with your forearm. Gently tip her up so that her head is lower than her tail and gently massage her crop. She may then sick up a foul smelling liquid. Only keep her head down for a few seconds at a time so she doesn't suffocate.
Yo may want to separate her from your other hens, and feed a soft diet for a few days. If you can get hold of any Avipro Plus, it contains all the probiotics which can help to re-establish her gut flora.
If, on the other hand, she has a crop impaction, her crop may feel granular, fibrous or there could be an obvious hard mass. Manual massage, two or three times a day can help break up the mass. This can be helped a little if you can get a little (just a few mls) of olive oil or similar into her. You may need to continue with the crop massages for a few days before the impaction completely breaks down.
Have no experience of it myself, but hope the above help - alison