Hajime's Demon Eye, Arifureta Vol 1

His Demon Eye couldn’t see things his normal eye could. Instead, it saw the flow and strength of mana surrounding a person or object, its colour, and the element and “core” of the spell they were trying to activate.
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Much like Sense Presence, Detect Magic as a skill only gave Hajime a vague idea of the position and amount of mana being used. It wasn’t much better than an alternate way of searching for enemies. However, with the enhanced Demon Eye, he could pinpoint the exact amount of mana being poured into what kind of spell.