Paint thinner (aka varsol) works good on oil based paints, and some others, but Laquer thinner works much better to remove just about anything. (just watch it around ANY plastic).
If that doesn't work, razor blades work great, but if its on the edges etc... a 3M green scrubbing pad beats them (you can pick up different grits at just about any autoparts/finishing store eg napa/partsource).
My personal preference now to buying 12" square pads is to stop off at Safeway, or Save-On, and get a kitchen HEAVY DUTY scrubbing sponge. Can't remember the brand name, but their about 1.5" thick, kinda oval shaped, and come sealed in a wrapper, their moist in the pacl with some sort of anti-mold stuff. These pads work even better then the 3M scrubbing pads, and don't (or haven't yet) scratched any glass I've used them on. I've taken off paint, that spray on bathroom frosting, dried egg

, etc.. and they haven't failed me yet.
Good Luck!
sd