It is not a good idea to generalize about hatches but I have some suggestions especially for the Eastern USA. If I wes fishing today, I would start fishing with the big swimming Slate Drake nymph because they are easily caught and eaten by trout. Drakes are starting to hatch but remember, they hatch out of the water. Only the spinners get on the water unless it is purely accidental. When the Slate Drakes, Light Cahills or Sulphurs are hatching, there will be a spinner fall late in the day. Often, you can catch more trout fishing the spinner fall faster than you can during the hatch.
If you notice some other insects hatching, that is coming off the water, it will most likely be Light Cahills or Sulphurs and it will be time to change to appropriate emerger, dun or adult imitations of the insect. In the evenings, Golden Stoneflies and Little Yellow stoneflies will be hatching. Remember that these species of stoneflies crawl out of the water to hatch so fishing a Golden Stonefly nymph or Little Yellow Stonefly nymph very late in the afternoon near sunset should be effective. However, . If you see the stoneflies depositing their eggs on the surface of the water, switch to the adult imitation of the stonefly.
If indeed the Light Cahills and Sulphurs,are hatching check the faster waters for the Light Cahills because they will be swept along in the faster water or riffles. The Sulphurs however, will pile up in the slower water of the larger pools.