If a hen seems "off", you will need to watch to see if she eats. Feel her crop to see if she has anything in it. And watch her poop for changes. Also, check everyone's crop in the morning before they have access to food and water. If any chicken has a full crop come morning, that chicken can be assumed to have a crop disorder that will need to be treated.
Meanwhile, try to think of anything a chicken may have encountered that could make her sick, such as rotting compost, stagnant water puddles, toxic substances such as moldy feed or moldy compost or mouse poison or soil under leaking machinery. Those things will provide clues as to how you will need to treat.