This question has been on endless rinse-repeat for decades. We viewers and fans KNOW it because we are in on the joke. The main thing is that Superman doesn't look like Clark and doesn't sound like Clark, because nobody really knows how Clark looks or sounds.
The glasses are a mere element of the Clark persona that Kal-el has successfully adapted since childhood in deference to Pa Kent's request to "keep it down."
Even Lois barely looks at him as she zips through work and assignments. Although she does notice "something" she dismisses it, not because she's too stupid, but because she doesn't really have enough information. It wasn't until Superman II that she actually gets a good look at Clark to see the physical similarity.
In the Daily Planet, Clark works in a large organization where his mild-mannered, fish-out-of-water persona gets drowned in the furor of hell-on-wheels deadlines and spitfire personalities. He even seems this lumbering idiot who gets stuck in revolving doors and knocks over wastebins on a probable daily basis. Notice, however, that Perry White recognizes Clark's snappy, punchy prose style, i.e. high-level of intelligence and writing skill, which is why White sought to mentor him with that little talking-to about guts and aggressiveness.
Christopher Reeve, gifted actor that he is, conveyed this duality best in that scene where Superman contemplates revealing his identity to Lois... change in posture, vocal timbre, the direct, confident gaze. Then, on second thought, switches back to Clark Kent mode with the stooped shoulders, soft-spoken stutter and eternal *duh* expression.
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