People with high intelligence have these traits:

They seem to have boundless creative and introspective thoughts on themselves and the world around them.

They tend to have problems such as addiction, loneliness/depression, overthinking/overcomplicating things and are often a bit misanthropic.

Either a ridiculous academic history of accomplishments (such as going to uni at an early age, or being a straight-A student with no studying throughout their academic life) OR an academic performance that seems to be extremely flexible and extremely unstable (such as only having high grades in a few key subjects while everything else is average or subpar).

Generally very competent in terms of learning new skills but often not being much of a master at many if any. Things like being good at multiple things are relatively common, like knowing 5 languages + a native language, but seemingly lazy and bad at 2 specific ones.

Good working memory and decent perception/insight into subtle differences that are often not people-related. Things like being able to notice that there's a new kind of instant coffee blend, but somehow completely unaware of someone's hair. Forgetting names but giving you a nickname like “blue shirt girl” or Blue for short instead of your name is an example of both points 5 and 1.

Being repetitive and often oversimplifying when explaining things however complex or simple. Like saying the same thing twice in different ways, creating scenarios for said examples, and things of that nature.

Easily bored or frustrated, even with what would ordinarily be considered complex tasks. Things like doing crossword puzzles but being annoyed throughout the process, often rushing through certain tasks and work like haphazardly fixing the dishes or just painting an entire room white instead of the striped pattern.

Not easily distracted or taken out of their head. To sum up: they don't seem concerned with the outside world and seem to enjoy often things of their creation rather than others. Such as insisting on creating a new board game because they're getting tired of one, and they don't want to learn a new ruleset instead rather create it.

Surprisingly carefree nature and casual level-headedness in stressful environments.

Highly opinionated and open-minded, yet seemingly apathetic.

Very aware of their personal space and often very deliberate in how they should approach it. Things like stating rules or mindsets others should follow when interacting with them (this might seem weird, and not as common a trait as you probably notice, but these are often unspoken and end up being followed by others).

Often very motivated to move upwards in one’s socioeconomic status or completely impartial or even unmotivated and even at times dislike power.

Proactive not reactive. In the sense that often things aren't happening to them and then reacting, often instead being that they act and then things occur which they then react to. For example, making popcorn, while most people wait for it to be done say in a microwave, in this metaphor the smart individual decides to do it on their own in a wok pan, rather experimenting and potentially learning something, than going with established routine or habit.

Usually successful in terms of finances, but generally seems to spend money less wisely. This might feel counterintuitive, but it isn't. Too lazy to explain or cite sources, not an obligation to do so anyway.

Seemingly lazy and arrogant, though often end up being surprisingly hard-working and humble once you get to know them.

Has friends and connections who seem to also exhibit traits of smarter people. A social circle filled with a bunch of physicists, philosophers, artists, and even athletes can be signs of someone with a high intelligence.