If you are talking about that bloody awful blue tape that is stuck to the frames and not the glass then you are welcome to the job!!
It's the most awful stuff I've ever had to get off a window, absolute nightmare.
If there are scratches on it, then it won't peel off, and it'll come off in itty-bitty little bits.
It isn't glued on, so there is no adhesive to soften.
I totally refuse to get the blasted stuff off now, just can't charge remotely enough to do it.
Compo on the glass is generally ok to get off with a decent razor knife (not a Stanley blade scraper, they'll do the job usually, but much higher risk of scratches)
but get a disclaimer signed, if compo (sand and cement) is on the glass then of course there is going to be a risk of scratches already being in place before you even start cleaning the glass.
If the plasterers have been clumsy enough to splatter the windows with compo and not rinse it off, the last thing you want is to be hit with a writ for scratched windows!
Best of luck!
Ian