Oh boy I thought that one I kept was a jack spring! It had as many spots on the bottom half of the tail at the top and the mouth was mostly black! The first wild coho I caught had a very white mouth so I figured this was a spring. I bonked it once I saw it was hatchery but it looks like I need to figure out some more identifying characteristics!
Is the best way to look at the gums where the teeth erupt? Black = Chinook? I looked at outer mouth and tongue area but not specifically the gums.
It takes time and experience. Don't listen to everyone who is saying it's easy. It's not always easy. That said - if you're not sure, let it go. Every time.
The easiest way to identify a coho vs a Chinook is a combination the following:
Mouth - Chinook mouth is almost all black. Coho mouth is black too, but the line along the teeth (especially on the bottom) is white, as you mention above.
Tail - Vedder Chinook OFTEN have a more heavily spotted tail (top and bottom), but not always. Coho OFTEN have a less heavily spotted tail (on the top or barely at all), but not always.
Back - Chinook usually have heavier/thicker spots along their back, sometimes they'll seem oval-ish or look a bit like kidney beans. Coho typically have smaller or less dense spots along their back.
Bottom line is, no single feature is foolproof. A combination of the above should help you be fairly certain though. And each of the above is a general statement, not a rule. There are many exceptions.