You can get Diatomaceous Earth at some really well stocked feed stores. It's also a staple in natural garden stores. Look for it in the organic section, as it's often used as a pest control in organic gardens. I think if a chicken has a really heavy worm load, you'll see shedding in their poop. They'll be feeling the effects well before that, though. Weight loss and anemia are good indicators, especially if you're still seeing them eat. As to whether the others would also have it, I'd err on the side of yes. Intestinal parasites are often found in their surroundings, so if one got a chance to eat the affected food source, they probably all did. (Tapeworm is a good case in point - they can easily pick this up from a free range session).
Edited to add: Using DE in the coop and in the run is a great way to keep the fly population down, I find. Although it doesn't have as much effect on the adult flies, it kills maggots.