Biggest giveaway is when the shelf is strictly "galaxy brain" material I.e. D+G/Nietzsche/joyce. They're doing it to virtue signal an air of intellectual superiority. Everyone who actually reads has guilty pleasures interspersed with more serious works.
For instance my current pile of books next to my bed is mostly theory and classic lit but there is also a heavy dose of my guilty pleasure which is high concept scifi and cyberpunk. No one strictly reads theory or history.