Keep in mind that particular wormers are only useful for particular parasites, so it is best to get a diagnosis as to which you are addressing. That way you will know which wormer will help their condition. Otherwise, you simply may be stressing their systems out by giving them a medication that does not treat the issue they have. For instance, tape worms and round worms are treated by different anthemintics (wormers), so if your chickens have tapeworms, using a wormer for round worms will not address the problem at all. And worse, it can conceivably make your birds sicker from the stress of medication they don't need. You will not want to worm your flock at all unless they actually have worms AND you know which parasites you are treating them for.
Your veterinarian will be able to perform a fecal smear and tell you what parasites your flock may be suffering from. Avian vets can be hard to find, but in many cases, any vet can perform a smear for you, whether they treat birds/chickens or not. Extension agents will sometimes make this service available, as well, and in most cases, you can find someone to do a smear for a nominal fee.
On a preventative basis, diatomaceous earth mixed in with feed at a ratio of 2% can reduce internal parasite load. If your flock has an acute infestation, it is not enough. Instead, these other dewormer products are also safe and effective... but do confirm the presence of worms and what type before worming your flock, so you can treat them with the right anthelmintic. Your extension agent or vet will be able to recommend the best medication for your flock's particular issue.
Finally, if your get your fecal smear test results back and your flock is clear of a significant problem with internal parasites, you'll know that if your birds are showing signs of illess such as a drop in laying, it must be due to a cause other than worms. Have a look at this list of other possible reasons your chickens aren't laying.