Internationally, "giftedness" is most frequently determined by a score on a general intelligence test, known as an IQ test, which is above a chosen cutoff point, usually at around the top 2-5%.

You have to take the 20-question test (conveniently placed on 20 different pages!) to actually find the cutoff scores.

The journal actually let the researchers of this article reference the online version of the test as a justification for the cutoff scores used.