How To Diagnose Velvet?

[FishWishDish]

Our goldfish have shimmering gold patches on them -- a black moor more than any, a calico oranda has a few spots, and the gold one and orange-capped oranda shine more silvery, which looks like natural shinyness. How can you tell if the gold (or silver) is natural coloration or if it's velvet?

If it's velvet, how can you treat it safely in a tank that has a pleco also? Malachite green can't be used with catfish, and the drop-in parasite tablets I have don't say they work on velvet...?? Confusing!!

Thanks for any help in advance...