It's often hard to tell with cats because they hide pain well. They might get a little more clingy or hide more often or that sort of thing, but it depends.
I had a cat who was basically a complete badass. Not completely unaffectionate, but she was very far from a lap cat. She was your basic slender, black cat who would walk along the tops of fences and catch mice and birds and climb the friggin' house at night in order to be let in through a window (she knew you'd be up there).
Around age... I want to say 16, she got into a fight or something and her eye got scratched and infected. Eventually, it just inflated into this giant crusty... mount... over where her eye should have been on her face. I don't know what sort of comparison to make for size, but it was a fair bit larger in diameter than a golf ball, but not quite that of a tennis ball. Turned out her cornea had basically burst, causing the above unpleasantness. Throughout all this, she managed to still live life pretty much as normal, albeit with a bit more lying down under a table fairly often. I can't imagine the cat could have been comfortable at all, yet you could just barely tell from her behavior.
Of course, the eye (or rather the horrible monstrosity it became) had to get taken out, and after that she was fine, although a bit more lazy and affectionate because of the whole ordeal, plus age in general. One-eyed black cat is pretty awesome, though.