Butterfly eggs are very small. The actual size varies between species but you can expect the eggs to be about 1-3 mm in diameter (like a pinhead or smaller). The eggs actually vary a lot in shape and color but you won’t see much of the variation without magnification. Fortunately, many of the common garden butterfly species that are easy to raise indoors lay eggs that start out yellowish or light colored so that makes it possible to see them relatively easily against green leaves.
Before starting out to find butterfly eggs it is extremely helpful to know which host plants are used by which butterflies, or in other words, what their caterpillars eat. Each butterfly species has only a few plant types which will provide the proper nutrition for their particular caterpillar (larva) so that is where you will find their eggs. For example, the Black Swallowtail butterfly lays eggs on the carrot family of plants such as fennel, dill, parsley and carrot. They also use the Rue plant. The Monarch butterfly host plants are the family of milkweed plants. The Gulf Fritillary butterfly will lay eggs on passion vines.
Once you have identified your host plants (or planted specific host plants), watch for butterflies flying around them. When you see the species of butterflies flying around the host plant to which they are partial, you will almost certainly have eggs and you may even see the adult butterfly laying the eggs. If you see the butterfly hovering or landing on a leaf for a few seconds then they are probably laying eggs. If you are close enough you can actually see the butterfly curl their abdomen down to the leaf.
As seen in these egg photos, many eggs start out light colored like an off-white to a yellow color then change to a dark color or black before the caterpillar comes out. Most caterpillars hatch out of the eggs in 3-7 days. It can vary depending on species and other conditions.
Some butterflies lay their eggs on the top of leaves and others lay them on the underside. In my experience, Black Swallowtails and Gulf Fritillaries tend to lay eggs singularly on the top of the leaves. Monarchs often lay them singularly on the underside. Most species of butterflies lay their eggs singularly but some lay their eggs in clusters on a leaf/stem such as the Mourning Cloak, Question Mark, Pipevine Swallowtail, and Baltimore Checkerspot to name a few.
Finding butterfly eggs is one of those things that gets much easier after you have spotted a few of them. Once your eye knows what to look for the eggs will seem to jump out at you. Your best bet is to have a plan to attract a particular butterfly to its host plant and start your search for eggs there.
Once you have found the eggs you can watch them develop in your garden or bring them in for the fun of watching the caterpillars grow and witnessing the incredible butterfly metamorphosis.
Butterflies lay a few to hundreds of eggs in their life span. The huge majority of them will not make it to become a butterfly. There are many predators and hazards in all stages of the life cycle of a butterfly. Out of a few hundred eggs, maybe only a few will make it to adulthood.
So, if you want to observe nature in action then you can watch them outdoors (which is also quite fun!) but if you want to make sure you see the whole miraculous lifecycle then you may want to bring the eggs indoors until you can release them as beautiful adult butterflies. If you have not done that before then I encourage you to give it try at least once, but beware, it can be pleasantly addictive!