... How do you spot cohos in the river?
Using polarized sunglasses (regardless of weather and lighting conditions) is by far your best bet for successfully spotting and locating fish that are sitting on or near the bottom, as the polarization allows you to see through the glare of the water's surface to spot and identify the fish and promising looking lies.
If you are fly fishing, I normally look for rising or "porpoising" fish in the "frog water" (slow-flowing or slack water) of the calmer stretches of the river, or search for deeper slots or pools that have slow to moderate flows. They can also be found near the edges and seams of the faster flowing water.
Hope this helps in some way...
Cheers,
Todd