As above, a lot of cleaners round my way don't even do the sills ! never mind the frames. So definitely mention that you do the sills, and frames if you're going to wipe them over in your clean. I get a lot of work because other cleaners don't want to include these things as standard. It's a tough world out there now, and adding that bit extra can set you apart and be the difference between success and failure.
I normally start with " Sorry to disturb you, do you have a reliable window cleaner at the moment ? " They'll either say yes we're fine at the moment / have a cleaner / had the same guy for years, or say " ohh glad you called, we really need someone"

or "no we don't have someone" then you can something like " would you like to know how much it would cost be to be added to my local round " (even if you have no local custies yet)
I'd say the best thing to remember to beat the fear (which I bet 99.9% of people have ) is that people are
always polite to you, they'll look a bit wary and standoffish when they open the door, but as soon as you mention window cleaning you'll see them lighten up as they relax and realise you're not pushing a sale of whatever..