One of the common ways to find a raptor that is perched is to follow the smaller birds that are harrassing the raptor, trying to drive it away from their range. Here is a Great Horned Owl Bubo virginianus being harrassed by American Robins Turdus migratorius.
With smaller owls, like a Northern Pygmy Owl small birds like chickadees and nuthatches are often doing the mobbing. With eagles or hawks it is often crows and jays who take the initiative to try to drive the offending raptor away.
In this photo I was trying to just capture the owl, but the mobbing robins were the real show.