Seems like it's since been reworked, but before 2014 elementary kids in the US were tested and potentially placed in the GATE Program[1] based on their results.

[1] https://www.cde.ca.gov/sp/gt/

Was school consistently easier for you than it was for your classmates? Was it much easier for you? How about compared to your smartest classmates?

I think the answers to those questions are pretty strong indicators. Fundamentally, though, IMO most people who are gifted know it.

Presumably you get told you're gifted by the education system following some assessment.

Personally, I'd never even heard of the term "gifted" until I left home for university, it wasn't a thing that existed in the rural location where I grew up.

"Gifted" is of course a spectrum. You'd know if you were smarter than the average bear if you got put up a class, put into "the smart kids class", or got selected for various leadership programs etc.

Of course not everywhere would have such programs, but there will likely be other tell-tale signs as well.

The most quantitative measure is IQ. People like to criticize it but in psychology it is the most empirically verified number that correlates with so many things.

>How do you know if you're gifted?

When I was growing up, it meant you scored 130 or higher on an IQ test administered by a professional psychologist. For me, I had this administered in 3rd grade. I assume some teacher suggested I get tested.