No, don't say "Do you want a window cleaner?" It's a closed question and they will invariably come back with "NO" because they haven't a clue who you are.
Introduce yourself, tell them what you do, tell them the areas that you already clean in, tell them how long you have been going, tell them how you clean windows, tell them how often you come around, explain the process, do you wipe sills, etc?
Then finally if they are interested, ask them if you could give them a quote for cleaning their windows.
Do the quote, tell them verbally but alsow write the price on one of your cards or flyers and hand it to them.
Be ready to answer if they ask when will it be done, how can they pay, etc, take names, address, postcode and telephone number or even email address.