Everyone "is intelligent" unless they have a severe brain disorder or damage. Many species demonstrate intelligence according to the dictionary definition. You'

Everyone "is intelligent" unless they have a severe brain disorder or damage. Many species demonstrate intelligence according to the dictionary definition. You're probably asking how to tell if someone possesses higher-than-average or exceptional intelligence. That's what IQ tests supposedly measure, so you could use IQ test scores. Without access to IQ test scores or something analogous that measures what generally gets called general intelligence or G, you have to observe how well a person solves problems, especially novel problems that require reasoning ability. That can get situational: a person may show high intelligence solving a math or programming problem, but get stuck trying to start a stalled car or frying an egg. A smart person can reason about and solve problems, so they should eventually get the car started and learn how to fry an egg using their own resources and something like scientific method (eliminate variables, simplify the problem, hypothesize, test, refine, repeat). I generally associate curiosity with intelligence. You can see that most easily with young children. Some seem curious about everything and ask lots of questions and try new things, others seem incurious. Adults may lack curiosity due to lack of interest, or focus on something else more important to them -- curiosity seems more situational with adults because they have more experience and preferences, and more demands on their attention and time. Even so, a curious adult willing to learn something new and take some risks (even just the risk of failing or looking inept) will strike me as more intelligent than someone who shows no curiosity. An intelligent person wants to learn. An unintelligent person either thinks they already know everything, or doesn't care to learn anything new.