The flat we lived in when we first got together was in an old part of the town and we had several mature trees... we knew there were owls in the area but never saw one... the last night before we moved here (Feb '92) surrounded in boxes we were both laying in bed and heard a male tawny calling (the classic owl hoot hoot - the female's respond with a "kee-wick") - we both sat bolt-upright and said "OWL!" and sure enough, in the big oak tree in the corner of the garden, there he was, silhouetted in the light, almost as if he was saying "been here all the time..."
If you know you have a specific breed of owl in your area I can teach you a neat trick, if you can locate a recording of the male calling... other tools you need is a night-light (a big torch with a thick piece of red plastic to shine it through will do - if you really want to show off - night-sights...!).
You play the recording on a loud "ghetto box" for a couple of minutes... owls are very territorial, and if a male is in the area, he WILL come to investigate, and then you can do the spotting with the torch without blinding his night-vision...