All I say is this
"Hi, I'm a local window cleaner in this area, just wondered if you need a window cleaner at all? I call around every week so you can have them cleaned at whatever interval suits you?"
They will either say yes or no, if yes get the details and arrange the work, if no, leave your card and ask them to give you a call if they ever change their mind, likewise if they say that already have a window cleaner.
Best wishes,
Trev