Remember that question about Tim and Nate fishing a pool on a river with a back eddy… where the trout were rising sporadically? Tim casts perfectly all over the run and nothing. Nate holds his fly, waits, and then makes one cast and connects. What did Nate see?
The answers you gave are good, from timing the clouds and shadows, to watching the rise patterns of the fish…
But the real answer is that Nate was watching the current seams and foam lines. When the foam line disperses, the fish disperse, but when the foam gets packed in a straight line seam, so too is the current packing the food (insects) together on that seam. Believe it or not, fish will key on that well-defined foam line. When the currents spread the foam, the fish disperse or wait, when the foam is packed in a well-defined seam, they eat.
This is a really deep cut from the guide’s bag of tricks. Guide Pete Cardinal showed me this many years ago during a trico hatch on Montana’s Missouri River. It works. If you time your casts with the patterns of the foam, you’ll make fewer blind casts, spook fewer trout, and ultimately catch more risers. Check it out for yourself sometime. When you find heads popping in a big pool, pause and watch a minute. Watch that foam, and you’ll see the feeding switch gets turned on and off as that foam and the currents pack and disperse.