2 things I have found with rainbows...
Residualized rainbows ALMOST ALWAYS have white tipped fins, and I've never seen a Steely with those (anal fin especially, see pic of resident rainbow below and Bigfishers pic). They have a more football shape, with lots of spots and also spots below the lateral line.
I have seen river rainbows as chrome or chromer than Steelhead, and sometimes they are the same colour, this is not a good thing to judge it on IMO.
My buddy also got a rainbow this year in the Veder - about 5 pounds - that looked like a summer steely except for white tipped fins (rosy cheeks and all).
Squirrel you say the thing that triggers a rainbows to smolt is genetics.. this is true, but I think you mis understood the question.
Why do some fish stay in the river 3 or 4 years before going out to the ocean?
Why do some stay in the Vedder until they are 5-6 pounds (have gotten a few in the past few years).
One other question that people might find interesting.
The last few years I have noticed a HUGE increase in the number of resident rainbows, has anyone else noticed this?
I wonder if this is a good thing, maybe meaning the river has enough nutrients to support a population without them having to go to the ocean for food?