This test isn't definitive, but if you are concerned it is an option to try it.
Move your hand quickly towards your cat's eye and see if it blinks. A blind cat will not blink. Test both cat's eyes separately. If the cat doesn't blink, it's time to see a vet.
A 16-year-old cat is prone to diabetes and it is very possible you may need to start giving her insulin shots, although if she is a diabetic, there is no guarantee she will see again.