The only reliable way I know is in late spring and summer, when grown chicks out of the nest call loudly and regularly from sunset till dawn. You can see them in torchlight. Be careful, however. Young are dudes, but their parents are shy and will not feed their babies in your presence. So limit yourself to a few minutes of observation and avoid places where many people go after one another. Also remember that some owls can attack a man fiercely around the nest.
Otherwise, it is hit and miss - walk or bike miles and miles over forest edge and grassland, and hopefuly you will meet some. Keep to dirt roads and tracks, because you are quiet and can cover more area. Animals are not shy of roads. Remember that animals at night, even owls, often see almost as badly as you, but are very sensitive to sound - trashing on dry leaves or saying a word will give away your presence. Remember that many daybirds call and fly at night, so a large owl is often a hawk which you just flushed from the roost. Sometimes roost sites are given away by dung and pellets under an old tree.
Overall, every encounter is magical, and I enjoy going at dusk to see both owls and night mammals.
