Others will know more about this than me but I'm pretty sure a worm egg count test will show presence of fluke. I thin it will be an incerasing problem with a wetter climate. The intermediate host for fluke is a water snail, hence the reference to wet pasture.
I use a combined wormer and flukicide - Fasimec Duo S. The blurb recommends using it twice a year for ewes in winter, prelambing, and in autumn. Replacement lambs to be done in the autumn. It has a 27 day withdrawal period for sheep.