i probably use a bell if there is one. sometimes you don't know if it works though. similarly, if there is a porch area a knock might not get through, especially if the living quarters are large and the casual room is at the back of the property. prefer to knock on wood (there you go) rather than glass though, if there is glass in the front door i mean. there's something to be said for just shouting through the letterbox as well. doesn't really matter