The closest you can get to drawing skin on windows is the sun-strip on some cars (and the writing in the rear side window, if there is any, comes from your number plate).
You can download skin kits for photoshop here http://www.barkingpig.org/msz/pages/kits.php. There is a layer in those called 'mask' or something similar which covers all teh parts of the skin that don't show up on the car, useful to see at a glance where you can and can't draw. There's also a wireframe layer which is very handy for lining up parts of the skin that are together on the car, but apart in the skin file.
You can also download a skin viewer which is much better than using LFS to check as you work, it's called 'CMX viewer' https://www.lfs.net/downloads