If you can't figure out which bird or birds are having the issue, then I would worm them all. Most adult birds will roost in the same spot, or nearly same spot every night, unless there has been a recent change in the flock. You can sometimes figure it out base on where the poop lies, under the roosts. If I suspect something going one with one of my birds I will often check with a flashlight where they are roosting, they tend to not move once it's dark. Then when I see the droppings in the morning I have a pretty good idea whose they probably are.