I would advise you to have a worm count done, its done using a sample of their droppings. Not cheap, my vet (through SAC) charges about 23 pounds per sample for worms and fluke counts. But it means that if they are free and stay on the same pasture you are unlikely to pick up any infection, and need to worm not very often. If you worm just because you think they need to be doen every months or so, you are likely to introduce resistance over time. If you haven't done a worm count since they arrived you should definitely do one to have a starting point (and also use the right wormer).
I had to worm the goats just after they arrived, and my pasture is free (so far). I also get my sheep done once a year now, have so far not had to worm then. Goats are free now too.