If you have marker on your windows you can use some Acetone Based Nail Polish Remover to remove the marker on your window.
Just take a rag and add some nail polish remover to the rag and then wipe the marker off the window and it should come off easy.
If not you may need to use some more nail polish remover and scrub the marker off the window using a little more force but just don't break the window.
Some undiluted vinegar can also sometimes work to help remove the marker off your window and it's not harsh either.
Rubbing Alcohol and WD-40 is also another way of removing the marker from your windows or any other glass.
Just spray the rubbing alcohol or WD-40 directly on the marker spot on the glass and let it sit for a few minutes then use a rag or sponge to scrub the marker off the window.