The signs are never as subtle as they think. The stupidest people are the ones who think they are smart

They get upset when you do something right.

The dumbest people are usually those who will never admit they don't know something.

Lack of self-awareness

The best way to tell if someone is stupid is if they consistently don't learn from mistakes. They don't seem to comprehend what they did caused the problem even after being told numerous times

“I'm right because I'm older”

The ability to ignore all facts and still claim they are right

They are NEVER wrong. Ever.

Zero curiosity.

Personally, I believe a person is stupid when they refuse to learn and willingly choose to stay ignorant. I'm not talking about people that are "unable" to learn, I’m talking about people that are able to learn, yet they choose not to.

Overconfident in a subject that they clearly know nothing of. And try to tell you you're wrong after facts have been presented.

They bring up their IQ score from an online test they took.

Sometimes you can just see there's nothing behind the eyes

They're unable to explain their opinions. They can't "show their work" of how they came to that conclusion.

Not being able to spell is normal, lots of people are dyslexic. Not knowing you can't spell, on the other hand...

They talk about how smart they are.

Lots of talking, not much listening. The empty can rattles the most

Honestly, as I get older, I realize more and more that some of those “subtle signs” really just reflect a lack of people skills or a lack of self-awareness. Sometimes a lack, of people skills or a lack of self-awareness, could be related to asynchronous development, development issues, or even mental illness. So sometimes, I try not to jump to the conclusion that someone could be “stupid.” That being said, I would say that a lot of stupid people, at least the ones who I’ve met, feel a compulsive need to put others down in the most vicious way possible. Those same people also can’t catch smoke when people respond back. A perfect example would be an ex-girlfriend of mine who would have no problem body-shaming any other woman in the room, but would go ballistic whenever someone teased her about gaining 1 kg.

Accusing you of sh*t they do themselves. When they're called out on it, they don't understand or can't comprehend.

They’re hawking pyramid schemes on their social media.

People who are proud of not reading.

I don’t know how “subtle” this really is, but: insisting that something isn’t real just because they personally haven’t witnessed/experienced it. Case in point: an acquaintance who was convinced that catcalling wasn’t as prevalent as people made it seem, because “well, I’ve barely ever seen it happen!” Like…yeah, bro, you’re 6’3”, bearded, and built like a defensive lineman, of COURSE people aren’t gonna be catcalling many women you’re walking with. Jesus.

Getting angry at novel situations or ideas instead of responding with curiosity.

Misjudge the outcomes of their actions. For example being surprised that they were fired after being a no-call, no-show at their work.

Not remembering when they held a different belief, literally actually forgetting that they held a different position. Smart people recall times they had inferior beliefs and they acutely remember the process of evolving to a better opinion. Smart people don't necessarily admit to being wrong outwardly all of the time, but they admit it to themselves.