If you are worried that you have resistant parasites, rather than just using deworming willy-nilly you should actually test to see if you do have problem. Every time you deworm, you actually help to build the chance of having a higher percentage of the resistant parasites because they are the ones that are going to survive a deworming. You want to only deworm when you NEED too and then ensure that you are using an appropriate and effective product.
To test for parasite resistance to a particular product, you run a fecal egg count prior to deworming and then again 10-12 days after deworming and compare the results. You are looking for a 90+% drop in the number of eggs from the pre-sample to the post-sample.
Licensed Veterinary Technician