Price them up which is a good price for you to do them, if you are cheaper than another wc without you realising then thats business. But dont undercut just to get jobs n the cheap, you will do them rubbish, regret taking it on, and actually give you a bad name when you disappear! Not only that but cause bad report with other wc in the area.
If you know on someone's house who already has a window cleaner if they ask you for a quote they are inviting you to do so, so give them a quote, if they decide to take you on its their choice.
Not everyone who has a window cleaner actually like's their window cleaner for whatever reason but they dont know who else is about so will stick with them. If your a different option, more professional in your job, appearance etc then you may be given that chance.
When handing out flyers i always say, If you already have a window cleaner keep hold of my flyer encase your current wc disappears, if you dont have one feel free to call me when you feel the need. I dont pressurise people and put them on the spot. They will always say No thanks and probably not even fully comprehend what you just said to them...
Just like in the town, someone comes up and asks you something, you reply - No Thanks without even given them a chance whilst walking by..
If you offer more services than just window cleaning, offer those too! they may need something else done, but not the window cleaning, then in time after you have done the work, they liked you approach and manner and give you a call to do the windows perhaps... Ive had that a number of times.