Very little is really known about the Upper Skeena steelhead populations, although by most accounts it is relatively low abundance. Most of the research indicates that the Upper Skeena fish tend to move through the mouth of the Skeena during periods of maximum commercial effort, which, no doubt has impacted their viability. If you choose to explore, you may want to shoot for a year with little to no commercial fishing effort. I suppose you could also try to get some indication by looking at the test fishery and noting how many steelhead are moving by in early-mid July, as those are most likely to be the Upper Skeena fish...an imperfect tool, but a tool nonetheless.
Whatever you do find keep close to your chest.