Window cleaning is a business like any other. If we were builders/plumbers/accountants/etc, would we worry about whether the prospective customer already used someone else?
Everybody has the right of choice, so if someone askes me for a quote, I don't need to know whether they already have a window cleaner or not. I give a written quote on my standard quote form, based on my estimate of how long the job will take, and any access issues etc.
When I hand them the quote, I point out my terms, which are that the price is based on the understanding that it is for a regular 4 or 8 weekly visit, not just a one off. I might at that point ask if they have a window cleaner already, and if the answer is yes then I will ask why they want to change. Depending on their reaction, I may or may not decide that they are the sort of customer I want.
I never worry about underquoting another window cleaner, as I charge top rates and it's unlikely I would be cheaper.
On the other hand, I would never do it the other way round, i.e. it's fine for someone to approach me and ask for a quote, but if I knock on a door and they say, "I've already got a window cleaner," I say "thank you" and walk away. If they're not happy with their current one, they'll call me back.
Cheers,
Ian