If you don't know what you're looking for finding an owl pellet can be pretty difficult. If you've never seen one you've probably overlooked it thinking
it's was no more than a small chunk of dirt. Since owls cannot chew their food it is pretty much swallowed whole. In their gizzard a pellet is formed consisting of undigested bones, fur, teeth, bills, feathers, etc. which they regurgitate. To find one look for areas where owls have been roosting. Look at the lower branches of trees and their trunks for "white wash" or owl feces. This is usually a sure sign of an owl roosting site. Also keep your ears open for mobbing by crows, which will pester any owl (especially the great-horned) they find roosting during the day and give away their hiding place. Search the area around the base of the tree for pellets.
Dissecting a pellet can not only tell you what a particular owl had eaten, but can also provide information of the types of mammals, insects, etc. in the area. The one I found (pictured) consisted of the bones of some type of rodent. Present are the skull, jaw bones, leg bones, ribs, vertabrae, shoulder blades, and teeth. I allowed mine to soak in water for awhile which allowed for an easier dissection. Be sure to wear rubber gloves and wash your hands afterwards when handling pellets. Salmonella outbreaks have been attributed to handling unsterilized pellets. If you come across tiny eggs, pupae casings, cocoons or larvae what you probably have found is evidence of a wool-eating or "clothes" moth. These moths have been known to lay their eggs on owl pellets. The developing larva feed off of the fur present there. If you're interested in dissecting an owl pellet but don't have the patience to look for one, you can buy them:
Mountain Home Biological
They sterilize them in order to make them safer to handle, but I would suggest you still wear gloves and wash your hands thoroughly after handling.