For me, there are 4 main criteria: 1) The ability to learn quickly and "enter the topic"; 2) The ability to solve complex problems (mathematical, economic, applied engineering, management and organization problems); 3) Ability to formalize and correctly formulate tasks; 4) Good memory.
Humor. What is the level of a sense of humor and everything falls into place right away. And my subjective is the intonation with which a person speaks, tells, answers a question, or asks you about something. As a rule, within 5-7 minutes you understand who you are dealing with.
For me, the main thing (hereinafter the importance in descending order) is the ability to draw conclusions from the information available . A thorough knowledge of a subject is not as impressive as the ability to quickly and accurately understand an unfamiliar subject with the help of a few apt refinements. Further, the ability to think globally, as well as the power and abstractness of thinking . Not everyone is able (without psychotropic substances)) to approach the idea of the infinity of the universe, the fourth (fifth, etc.) dimension, the world from the point of view of solipsism, and so on. Yes, the same concept of attraction of objects due to the curvature of space (according to the general theory of relativity) - if most of your friends understand how this should work, then you are in luck. At the same time, it is not necessary to accept these theories, but it is amazing when the imagination and operatives are enough for such assumptions. Impartiality. Following \ not following the mainstream does not matter as long as a person can justify his / her so. Let at least Kant, at least Mayer re-read on long winter nights, if he can convince those around him that his choice was made not at the behest of his left heel, but on mature reflection using fair arguments. (it is not necessary to convince, the main thing is to have such an opportunity) "Subtle" humor - appropriate and forcing you to think a little. Education. In any form, self-education is also great. This point will seem controversial to many, but what to do. If an uneducated person for some reason (which is unlikely) has all the other qualities, then he is not so much smart as witty and perceptive. Perhaps his childhood was spent in an environment actively developing the brain, or genes curled up into such a lucky cookie, or something else went wrong ... But the mind is a complex and complex concept, which, in general, gives rise to problems with its measurement. Without a certain level of knowledge, the mind does not exist, but remains only ingenuity.
I really want to write a huge list of "smart person, this one who never ...", but it would be too categorical, and therefore, that's all.
An intelligent person is able to analyze information, draw conclusions and make decisions. The more useful decisions and conclusions are, the smarter the person is. This includes the ability to formulate your point of view, and making reasonable life decisions, and building harmonious relationships with people, and success in the professional field.
First of all - by whether he is able to independently and rationally argue his point of view in the course of direct dialogue. On your own - it means not relying on quotes from authoritative sources. Rationally means not appealing to the emotions of the interlocutor. To argue means to explain in the necessary details why a person thinks exactly as he thinks.
If a person cannot do this, he is, alas, stupid. If a person does not want to do this, he is, with a high degree of probability, also stupid, since manipulations are effective either in a crowd or in relation to addicted subjects.