Unless you're packing a microscope to the river and you know how to read fish scales, which by the way is quite an art, you'd never know how many times a fish has spawned.
When you read a steelhead's scale to determine whether it has spawned before you will see a pattern of rings, much like a tree. When the fish is in the ocean it is relatively living stress free and the rings usually are a little wider apart but when the fish enters fresh water stress begins to set in because the fish stops feeding, etc. so the pattern is broken. It is usually thinner or darker. When and if the fish makes it back to the ocean the pattern changes back to the stress free pattern. When the fish comes back to spawn again the pattern will change once more and so on. It's quite a science and you'd need someone who specializes in this to show you the changes in the pattern because it's not that easy to detect.
Steelhead in the Copper R.( located below Sandspit on the Queen Charloote Is.0 have a very high rate of multiple spawning. One in three comes back for a second spawn. I don't know if Fisheries still does it but they used to have to set up a trap to stop the mending kelts from getting back to the ocean in May because there's a early run of sockeye that comes into the Copper during this time and the Natives net them. If the nets were set up on the weekend the traps would be employed so the kelts couldn't be netted. After the native netting was done the traps were opened to allow the fish to get to sea. I wonder if they're still doing this? Anyway, hope this answers your question.