If their feet touch the ground, they'll get worms eventually. You dont have to wait to see worms in their poop, if you do, that means there's an infestation. The couple of worms that you see in the poop means there's hundreds inside them laying thousands of eggs being shed which are deposited on the ground to be picked up again by a chicken to start the lifecycle all over again. It also means there's no more room in their innards, that's another reason the worms would be excreted.
If and when your chickens become lethargic, slowed eating and drinking and not laying...most likely it's caused by worms, lice and mites can cause the same symptoms too. But it's easy to inspect them for lice/mites. ETA; yes a sample can be taken to a vet to be checked for worms and cocci. The vet can tell what type of worm they possibly have and you can treat accordingly.