I knock every door. I tend to find the dirtier the windows the less likely they are to want a window cleaner as that's the way they live.
I find the easier ones to book are those who have clean downstairs windows, but their upstairs are dirty ... it shows that they like to have clean windows but haven't got a window cleaner. Also if they have slightly dirty windows but perhaps look after their garden, or have nice clean cars on the drive, it shows they look after their property but haven't got a window cleaner, so a more likely to book.
With that said though I book jobs every day where the windows are clean, maybe they clean them themselves or could be unhappy with their window cleaner. It's impossible to judge, which is why I knock every door.