How does one tell the difference between not being able to see, and just not noticing?

Eye test !! Have your eyes seen to by an ophthalmologist or at least an optician.
Vision testing. This is a somewhat metaphysical question- if i don't notice something, have i truly seen it? But, medically, the most important thing is to evaluate the vision function- essentially, to be able to visually resolve a standard image in the way we expect. Visual testing is straightforward; the rudimentary test can be done at pcp, while an ophthalmologist can test in more detail.