Fairly simple to know if you have them, dip in revive the colony you're suspecting the most having them and you'll see them fall at the bottom of the container.
The normal way of dealing with them is periodic dipping, but that's a pita.
I deal with them right now and I'm having some success with zeovit flatwormstop. I think you still need to dip your colonies to help, but not as much. I've dip the original colonie that was affected and I think it is now free of them after like 6 weeks. When I bast the corals I can sometime still see 1 or 2 coming from some corals, but it seems the population is falling. The product by itself seems great for colors and polyp extension. I do double dosing and didn't have any algae or cyano increase. I'll probably keep dosing it permanantly (I only have 25g, so it's not that expensive for me). Apparently Thomas Phol was recommanding to stop dosing iodine while dosing flatwormstop, I guess it's reducing the slime the coral is producing.