Butterfly and moth eggs can often be found by simply by inspection of the food plant durning the appropriate time of year (see egg laying timeline).

For example the Viceroy always lays a single egg on the upperside tip of a leaf.

Monarch eggs are usually at the top of the milkweed plant near the flower head, OR at the base of a leaf on the lower part of the plant. Female Monarchs choose young tender milkweed plants more often than old tough plants

Checkerspot species place their egg masses on close to the bottom of a plant then the caterpillars by climbing upwards eventually reaching the top of the plant (e.g. Baltimore egg location).