Velvet shows first as rapid breathing, often before any dusting shows on the fish's body. The fish breath fast, hang in the current and soon stop feeding.
With ich, the fish show larger spots, and these coalesce into more of a white coating later on in the infection (where people mistake it for velvet) but the fish will keep on eating, right until near the end.
If the angel is breathing less than 120 beats per minute (take a video of it and count from that and then multiply it out for 60 seconds) then this is ich.
Jay
No - that is mistaken advice, used for freshwater Oodinium, a different species.
Jay