All chickens have some worms, and as Rick says, these are not visible to humans in the early stages. Regular twice-yearly dosing with flubenvet for a whole week, with no other food, will ensure that the worms are kept to a low level which the chickens can live with. If the worm burden gets too great, the bird will lose weight and condition because the worms are getting all the nourishment from the food they are eating. Egg laying will slow down or stop, and eventually they will die.
If the hens can be kept on clean pasture, or if you pick up the poo in their run, you will lessen the risk that they will quickly become reinfested. Worm eggs live in the chicken's droppings, and can get eaten again by the chickens when they scratch around, or when they eat earthworms which contain earth contaminated by chicken worms. When this happens, the worms life cycle is complete, so yoiu want to try to keep their run as clean of poo as possible.
All the posts about worming are now in the new Pests and Predators section, where I will move this thread.