Get a FEC done, by Vet or by post. Your vet might know more about the worm & fluke history of the area too and your neighbours.
Are you breeding sheep? i.e. do you have lambs? If you aren't then your FEC count will probably come back very low and you probably don't need to do anything (this isn't guaranteed so get the FEC done). If you have lambs then it might be a different story, as they are more prone to worm infestations as they haven't yet built up a resistance.