It occurs to me that I could use the following simple rule I made up myself. If it is a hatchery fish with the adipose fin gone then its probably a coho. If it isnt hatchery and doesnt look like a pink, then maybe its a Spring.
*Most* hatchery clipped fish are coho but not all. There are hatchery clipped springs and apparently even sockeye, so this is not a foolproof method but you would be right 95%+ of the time.
Look at the head/mouth, back, and especially the tail and you will be able to discern coho/pink/springs 100% reliably. The spots are quite different once you see the difference.
Chum, look for the tiger stripes which are evident on essentially all Vedder chum.
Sockeye can be tricky to ID but look for a lack of spots. Coho/pinks/springs all have easily seen spots on the back.
If you are not sure, then let it go.