Determine if the plants on which you discover the eggs are mustard plants. If so, and if the eggs are located on the flower stems, the butterflies laying the eggs may belong to the Anthocharis sara complex.
Look for eggs on flower buds. These will often belong to the genus Euchloe.
Euphilotes ancilla females, on the other hand, often will lay their eggs on flowers.
Look for eggs on the upper or lower half of the plant. Certain females, such as the Anthocharis thoosa thoosa, prefer to lay their eggs on the upper half, while other females, such as Pontia sisymbri and Phyciodes mylitta, target lower leaves of the Descurainia pinnata and bull thistles, respectively.
Seek out eggs on the tips of tree leaves, such as oaks, cherries, and cottonwoods. Frequently, these eggs belong to particular kinds of butterflies, such as viceroys and cardinals.
Seek out egg clusters or strings. Frequently, you will be able to identify butterfly subtypes from the color and nature of the cluster. Hackberry butterflies lay small clusters of white eggs, while Neophasia menapia butterflies lay green strings of eggs on Pinus edulis trees.