If ever there is paint or cement etc. on windows before I touch them I explain to the custie that it's not "normal"
for me to have to remove such mess.
I stir it up a bit by saying whoever left the windows in that state (painter, carpenter, brickie) should have cleaned up after themselves and never walked away leaving the job in that mess.

However I can help the custie out of this predicament by removing the mess which will require the use of a scraper.
It will take extra time to do and will have to add this "extra" to the price of the job.
I also tell them there is a very small risk that the glass could be scratched.Then I wait for their reply.
99% of the time they tell me to go ahead. As long as it's done properly everything should be fine but I have covered myself by telling them of the risk upfront.
Any other scratches/damage which was there previously should be highlighted to the custie.