I noticed that a cashier at a grocery store was wearing fake glasses that had no lenses. The bright lights shine on glass and those reflections were absent, that's how I knew. It looked weird, as if she had part of a Halloween costume on. I guess she tried to look smart.
You can also tell if the glasses have much correction in them. The line of cheek breaks if you look at the face from an angle and the eyes can look smaller or larger. Clark could get away by letting everyone assume he only has mild corrections.
I could not fool people by ditching my glasses but I have so strong mannerisms. If I were a disguised superhero, I would avoid the same people meeting me in both guises and especially those that know me in civil. I would not stay long to talk to people nor pose for photographs. I can be fooled, I am prosopagnostic. I might notice your voice though.
Superman doesn't look like he has anything to hide although a tabloid reporter might want to know where he goes after rescuing people, who he dates, where he sleeps or if he sleeps at all etc. Also, there are millions pairs of eyes who could notice something. Just by being observant or lucky.
Some ficcers have Clark so attached to his glasses that he won't remove them when he goes to bed if unaware Lois is present. I don't know how non-bespectacled people see it but if I saw someone go to bed glasses on I would think something is definitely up. I take them off because it is more comfortable and I fear my frames will bend when I snuggle my pillow.
