Digital BW, The Print

Most manufacturers of inks put a "use by" date on their cartridges. 
There has been no discussion of what happens to inks when they get 
old in the cartridge. Manufacturers don't talk about it either. Is 
there a common phenomenom that happens to inks when they get old? In 
other words, say I have an unused and unmarked cartridge laying 
around in a drawer. How could I tell by printing if the inks in this 
cartridge were old and no good? Anybody run into this? I have a 
cartridge that's pretty old (8 months) and it seems to be changing 
the way it prints, and I'm wondering if its shot due to old age. I 
don't know how unused ink typically changes as it ages.