b11... that's a great question, and a great idea, but I think I've got bad news for you.
These Lexmark carts have the print heads built into them... this means that there are no NON-OEM cartridges available for these models... the cheap cartridges are refilled / re-manufactured. Since the only ones that make these carts is Lexmark, you're unfortunately stuck with the carts they make, and unfortunately all their carts are non-transparent plastic.
Out of frustration I tried to create a window into my cartridge with some plexi glass cut to fit in the cartridge. Well, after a lot of work and a ton of ink all over, all I was left with was a leaking cartridge and a big mess.
I've heard of others getting a window to work, but my best suggestion is to just top off the cartridge with ink when you think it is at about half full. Toping off is a GREAT idea for many reasons including keeping the sponge from ever drying out and the insurance against the print-head overheating from lack of ink.