Most cleaners insurance policy wont cover for the glass thats being worked on at the time of damage unfortunately.
I badly scratched a customers pane of glass a few months back, it really was quite bad, but was obviously done by a scraper-blade (back and forth, unparallel lines in the area of about a large stamp; NOT over the whole pane or anything like that). I was willing to pay for it to be repaired but he was fine about it, theyve since put a sticker over it.
Sorry, what Im trying to say is, and Im sure you know this anyway, but a manually made scratch created from trying to remove cement or bird doo-daa is obvious, by the lines left.
So in reply to the question, by a sticker with 'We do not buy goods or services offered for sale at this address' on it.
Matt