One way of telling how healthy your chickens are is by their droppings. They should be firm and dark but 1 in every 10 droppings will take on the appearance of chip shop curry sauce. This is called caecal poo and is just an indication that the chickens gut is having a good clear out.
Loose looking droppings like diarrhea can sometimes be caused by a chicken eating too many slugs and worms but it’s best to keep an eye on them to make sure they’re not poorly. Their vent (bum) should be clean and clear too.