It's trial an error unless you know the region of collection. Also, knowing what type of lighting they were under in the persons tank you got them from also helps. If they came from a system with high light, I would place them half or 2/3 off the sand. I don't believe polyp should ever be placed just under the water surface.
If you place them on the sand for a week, just watch and see how they do. Then move them to mid tank levels, what for retraction, fading and good expansion. If all is well, I'd leave them there. Persistently moving them around will only stunt their growth. Hope this helps.