It takes some knowledge. But it's relatively predictable in that the same bugs hatch from the same places at the same time of year at the same time of day (and deviations can be predicted by weather). Every bug is different.
Trico's, as you likely saw, generally hatch overnight, go to trees, molt into spinners, then take their mating flights and fall dead in the mornings. From July till first frost, mainly on slower limestoners. On a hot, sunny day they'll be early, pretty much when the sun hits the water. A cooler overcast day, or as you go into fall, it happens later and is often more spread out.
Many of our mayflies are springtime bugs. You have the Baetis variety olives in February-April, generally hatch midday on limestoners. Transitioning to hendricksons and March Browns - daytime hatches, late evening spinner falls. To sulphers - which hatch in evenings and fall in late evening, to green drakes at the very end of May or more typically early June. One thing to know is that most of the hatches start off at midday in like March time frame, and progressively the action moves later and later into the evening. A storm or something can start things earlier, but by late May it's a fish at dusk till after dark and come out by flashlight game, especially if a sunny day. Then the blue quills of late June through the Trico's of July-October flip the script and make it a morning thing, moving progressively later morning as you get into fall. BWO's come back for a midday fall run before winter shuts things down again.
Not every stream has all types of bugs, of course. And most bugs that are "about to hatch" will start a week or two earlier in some streams than others. That you learn from hatch charts and experience. When the sulphers hit Valley and Tulpehocken, they'll start bout a week later on Penns and Spring, and another week later on BFC, Kettle, upper Pine, for example, and even later if you wanna go up to the Adirondaks, you can follow a hatch all around.
Likewise water type. Their are clinger, swimmer, and burrower mayfly nymphs. A clinger like a March Brown will mostly hatch in heavy riffs. A burrower in silty slow water. A swimmer in the glides. Most spinner falls, though, fall above riffles (but often current feeds them down through the pools).
