Fenbendazole isnt used to treat coccidia; corid or sulmet is used for that and yes, a stool sample is the best way to identify it. In both your cases as I stated earlier, the dosage or amount of fenbendazole your vets gave you may not be as strong as what others purchase at a feed store...which is an equine paste wormer. That is why you were given a 5 day dose. It takes longer than 5 days for larva to hatch inside a chicken...that is why we go back in 10-14 days and reworm with a different wormer such as ivermectin or valbazen to kill the larva and other types of worms and prevent immunity to one certain type wormer ie...fenbendazole. Normally for first time worming I highly recommend wazine to get rid of the large roundworms to prevent worm overload toxicity, then reworm in 10-14 days with a different wormer. Then the next cycle, use different wormers in rotation to prevent worm immunity to any given wormer.