if I find a mayfly nymph and want to know if it will become a BWO or Cayhill is there an easy way to do so.
Not really. In large part it's because a BWO can come from several different types of nymphs. Cahills usually come from clinger nymphs, but there are many types of those, and some aren't called cahills once they hatch. There are several common names for each species and several species for each common name, which makes these kind of predictions hard. Check out my editorial on
Latin names to see more about the complications common names create.
There are a FEW things you can easily say. For example, if the nymph is a long yellowish burrower, it's going to hatch into some really long-bodied mayfly, like a Hex or a brown drake, yellow drake, or green drake.
Still, that's not terribly helpful. It's much, much easier to just learn to recognize the major species from your area. You've hit on one of the main reasons for doing so: because it helps you figure out what a nymph will hatch into. Or, if you have an adult, it helps you figure out what kind of nymph to fish while it's hatching.
Hopefully this site makes them a bit easier to learn.