I believe velvet happens when water conditions are less than desired. Mine broke out when my salinity and nitrates were not soo good. How have yours been over the recent period? How old is the tank and how long have the fish been in there? New additions? Grouper in particular.
Look for fine white dusting on the pec fins, That's where I think you'd see it first. But work fast if you think it is velvet. As Shark said, it kills fast. In SW
fish, it starts in the gills and spreads to the body, so when/if you see it, it is already late.