Dogs are the easiest way, cause they will (if trained) jump on and bark at the actual tree the squirrel is up. Second easiest is to find a big hollow tree, and sit within shooting distance of it and wait for them to come out. Next would be walking through the woods, cause squirrels usually freeze or run in a hollow tree when they hear you coming. They can be picked out while walking, but it's more difficult without a dog helping you atleast get close to the right tree. Another advantage the dogs have is if the squirrle hits the ground alive they can generally kill it without getting hurt, and without tearing it up as bad as trying to shoot it again would.