First, you need to observe what s happening around you. Put on your Sherlock Holmes hat and do some investigating, maybe fifteen minutes worth. Take a look at grass stems and weeds near the shore line for clues of a recent hatch. Stonefly nymphs crawl out of the water to hatch into adults. This transformation occurs on weeds, grass, rocks and anything else handy near the shore line.
Mayflies molt after they hatch.
This also occurs on grass and weeds. While you look for clues of a recent hatch, see if any aquatic insects are crawling around on nearby bushes. Streamside brush is a great hangout for aquatic insects that have recently hatched and are waiting their turn in the egg laying cycle. If you see a lot of a certain kind of insect hanging around the brush, you can bet on patterns that imitate that insect when you get to the stream. Spider webs are a great place to look for clues.