Well for starters, if you just got them, you need to give them some time as they are getting used to not only you but their new home. It will take time for them to start eating just be patience. Now flagellates can be identify when they poop. Most people would tell you that if they poop white segmented that would probably be the sign, moreover if infection is present then the fishes would be emanciated and super pinch. (Very skinny) only then is when they would refuse to eat. And in most cases almost impossible to bring them back. Flukes present themselves usually when you see your fishes scratching against objectsl, but sometimes discus scratch too due to other factors, so scratching isn't always the best way to identify flukes. When flukes are confirm and are in an infestation form, usually the disco would stop using one gill and breathe fast using only one. So take time and investigate to see what it may be. HTH