Test Before You Worm
It is custom for most horse owners to worm with a strong dewormer in the spring to kill anything. This was how it was always done. However, now there is an easy way to test the fecal of the horse and see if there are any worms and also sand. Simply have your vet take a fecal sample when they are there or drop it off to the office. The cost is usually $12-15 to have the test and then you know for sure. As a bonus you can also see if there is any sand.
Once you get the results you can worm accordingly. If there is moderate or higher sand amounts you can give a supplement to help clear out the sand. I have found this method is much better for the horse's health because you are not overwhelming the body with a dewormer if it is not needed. This is the route that I take. I test twice a year once in the spring and again in the fall. If your horse is having diarrhea or not feeling well you can always check the fecal more often.