They will be infested long before they get to the stage of having visible worms in their droppings. When you see worms, these are adults which are being shed, and they will have deposited hundreds of eggs in the chickens gut before departing.
If I had as many chickens as Foxy, I would certainly get them poo tested before worming, because Flubenvet is quite expensive stuff for many birds, but with only 4, I just do them very 4-6 months with Flubenvet on a routine basis. It does them no harm, and last year, when Emma Nelder was doing her poo sample dissertation, she actually tested my girls before and after worming. Before, only Marigold had measurable worms, don't know why, but after, they were all clear. So it is possible for a small proportion of the birds to need worming, and I feel that doing everybody routinely helps to keep the problem under control as there will be fewer eggs dropped on the run and the grazing for them to re-ingest. Daily poo picking in the run helps a lot, of course.
I think it's a bit like red mite, no need to get paranoid, just be watchful and have a good preventative programme in place.