Start by picking up a rock. Turn it over. If you have done your homework, you can tell if those are mayfly nymphs or cased caddis. If you know your nymphs from studying, you can tell what insects and what stages are active. Make sure to bend over and look very closely as to what insects you can spot on the water.
Spinners are almost invisible at times.
Walk the bank and shake bushes. Take a seat on high ground and watch the water. Don't fish for five minutes or ten, if you're patient. Begin by watching the insect lighting on the water on a section of the water. If you can see trout feeding, you can often see the white of their mouths opening to take food.