When you're new to PS salmon fishing any info is helpful....and almost all of it is confusing. What I've learned over the past several years is that there is almost no easily discernible single certain feature to rely on. The most trouble I've had is with spots. The most reliable has been the black mouth... but even on that I've had several coho that had remarkably dark gums...so even the 'white mouth' on coho can be misleading. What I've learned is that you have to take into account as many factors as you can. As an example I've attached a photo of a tail of a Neah Bay small Chinook from this past week - notice no spots at all and it's quite silvery. The ~9lb fish had a completely black mouth and did have some, but not many, spots on its back. For me, it's even tougher to descriminate resident coho from SRC. There's been several threads on here over the past several years on that difficult topic. I think the decision tree helps... but should be looked at as a guide and not certainty.