I will disagree with the statement that you have to deworm all of the birds. I agree that they may all have parasites if some of them do; however, an individual animal's susceptibility to pathology from those parasites differs greatly. 80% of the worms are contained in 20% of the animals in any given herd situation. I'm speaking largely from hoofstock (goats, sheep, horses, cattle, etc) perspective, but logic suggests that it would be similar in grazing birds. Unless your birds are completely confined and never in contact with the ground, you will never be able to eliminate 100% of parasites. The idea is to eliminate the worst infestations, to limit the numbers present in your flock. There are no recommendations on how to do this in chickens, but something to keep in mind.
Veterinary parasitologists are rare, and the new recommendations for deworming ruminants and horses are just now starting to be adopted in many cases. Some forward thinking people started 10 years ago, but it's still new to many. I propose that we should start doing similar things with our flocks of chickens.
Testing should be always available, it just depends on how dedicated you are to doing it. I personally would select anyone that may not look as healthy as the rest, and then a few more samples, based on how many birds you have. It is simple. When a bird poops, put it in a plastic baggie. Keep it in the fridge until you can take it to a vet clinic. A couple of days to a week is the longest it might be considered ok to test. Any vet clinic can fill out the forms and send it to a lab if they don't want to run the sample themselves. If your local vet refuses, you can call your local agriculture extension office. They should be able to direct you to an appropriate laboratory. Some states require a vet to be involved, but again, your vet that doesn't treat chickens can still send in a sample.
See the following link for transcripts from a recent public meeting at which veterinary parasitologists discussed this topic in ruminants and horses only:
http://www.fda.gov/AnimalVeterinary/ResourcesforYou/ucm318015.htm
There isn't any science to directly support that the same is true in chickens, but the biology is not that different.
Food for thought.
