Good that you are not finding external parasites!
OK. Anatomy photo below. The Crop is located on the right side of the breast. It's like a pouch that holds food/water. During the day as your hen eats/drinks you will be able to feel the pouch "grow" or get bigger. Sometimes depending on how much she eats, it may be very obvious as you just look at her. But you can take your hand an feel downward on the right side of her breast and feel for the crop. It shouldn't hurt her or be uncomfortable for you to touch her (She may be shy or not used to handling, that's another behavior).
Most hens fill their crop at night before roosting, you can feel of it then if you wish, usually that's the "fullest" time. As she sleeps, the food is processed through the digestive system. Birds poop a lot at night. In the morning if you feel the crop again (before she eat/drinks) you should feel that it's flat or empty.
Since you have other hens, practice on them if you wish. I used to run my hands down their breast quickly when they went to roost, then first thing to become familiar. Now after all these years, most of the time I can just look at them and see if they've been eating/drinking, the crop size and if it's empty in the mornings, but if one concerns me, I certainly do check.
Canker. You'd open the beak and take a look inside for any yellow or white lesions/plaques. There would also be a foul odor. Canker is not that common, but with an unknown history, it's best to check. Canker is contagious, but it can also cause plaques in the airway and esophagus which can cause some respiratory symptoms, gaping, etc.
Deworming is a good idea, even if it's not Gapeworm, you'd want to eliminate common worms like roundworm, cecal worms, etc.
Are you in the U.S.? If so, then follow these instructions.
To treat most worms that poultry can have except for Tapeworm you can use Safeguard Liquid Goat Dewormer or Equine Paste (Fenbendazole) or Valbazen (Albendazole). Both are given orally by weight; they do not mix well with water.
Safeguard dose is 0.23ml per pound of weight given orally once a day for 5 days in a row.
---OR---
Valbazen dose is 0.08ml per pound of weight given orally once, then repeated in 10 days
Here's how to give oral medications:
https://www.backyardchickens.com/ar...dications-to-all-poultry-and-waterfowl.73335/
If you're struggling with video, you could try using the BYC Gallery, just let us know where to find it.
https://www.backyardchickens.com/threads/how-to-upload-videos-to-the-gallery.1631126/
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