How can you tell the difference from a true impacted crop and one when the organ shuts down? I recently had a pullet with impacted crop, I took her to the vet and she flushed it. This is my first experience with this problem (only three years keeping chickens). She is doing fine now, however since it took me a while to figure out what was wrong with her, she had a very bad infection that was treated with antibiotics. The vet bill went sky high so at this point I would like to be able to determine if it is a true impacted crop or something more complicated. I also have Marek's in my flock (found out last spring with a necropsy) and I read that impacted crop could be one of the symptoms.
I guess what one could do is what you recommend, which is isolate the bird and stop any food for a day, just water with organic ACV, massage several times a day, give some plain yogurt the second day and hope for the best.
Sometimes, you just don't know until you clean out the crop and see if it's going to work again. What I recommended was for sour crop, mainly, not impacted crop, though it can help an impacted crop as well, if it's not too severe.
Marek's causes all sorts of things to not work well again, leaves them open for opportunistic infections, etc.