We are seeing more and more of these on builders cleans.
If render has got on the stuff then they may well be buggered. Mr Sheen on to a rag and wiped over the frames can be a good way of dealing with those affected by render or compo as it helps prevent them drying back white.
Solvent shouldn't really be used, but can be very lightly, as long as you know what you're doing and take care.
Magic Sponge / Eraser, same as solvents really.
Regular glass cleaner, either squeegee applied or by a spray bottle, depending on type of cleaner being used, will work if it's just dirt you want to shift.
Finally, if slight chips etc. then you can get a touch up pen.