Intelligence is much more multidimensional and far more fixed than we normally think. Multiple intellectuals are not fixed or natural. This is partly the result of our complex neural system. People with fixed belief systems tend to have "lower intelligence" than can be developed.
However, if we are talking about general, general knowledge, here are some potential signs of a person that I will look to see if a person is "smart", despite the complexity of intelligence:
Being broad and open-minded on a wide range of topics. If you are narrow-minded, you will have a hard time increasing your "crystallized intelligence" by magnitude.
Introspective and curious. Usually more listening than talking. It can be in the form of sensory search and in some "geniuses", they can be counterintuitively "impulsive" (Edit: Yes, I realize this may seem contradictory to some items on this list, people rational has a different personality).
Developing explanations and understanding of how the world and people work. It can handle the strongest arguments of both sides and take on an original controversial position, if it has any real merit. If you can't describe the argument of the other side, you do not know the subject!
Strategic planner, strong problem solving skills. Specifically in the novel or relatively familiar situation.
Accuracy in information analysis is available. Can be shown as a perfectist in getting the meanings right, but needs to be seen in a broad way.
It takes a lot of time to understand metacognition, monitoring a person’s course of understanding.
Has a strong, unusual memory. It is often underestimated by people who take pride in strong logical-spatial skills which means strong in Physics, but often weak in other "more complex, uncomplicated" topics and do not recognize the this weakness that is severely impaired in the development of their intelligence, imagination and creativity. A wide range of long-term memory items have been proven to enhance creativity beyond the “bound rationality” of short-term memory.
Does not engage in black-or-white thinking and can display objects in granular form. Often other people may be frustrated that "too much information" is being displayed, "you are thinking too much about things", "you are too serious" etc., until this person starts to know how to really simplify, compress things (often in unique ways), sometimes in an attempt to seem less good and / or get along with peers. However, if this person can go up and down the levels of abstractions when others seek advice, they can do so easily.
Recognizes what is unknown, but one “knows also the things” that are true without having to know “how” and / or validate every single point in depth right away. Can make a wide range of assumptions in their head, and discard false assumptions quickly once new information is presented with flexibility.
Unusual awareness of world events at an early age can be a sign. I find it quite reliable, but it can only be useful in specific circumstances.
The method of questioning can determine how many dimensions of thought are used when asking, and how a smart person can narrow down the topics, so they do not waste time.
Common in intellectual feelings, emotions can be separated despite loud noise, disaster, or personal trauma. Intellectualization is a two-edged sword that can create troubled behaviors and / or feelings from endless decay.
It is usually easy to experience boredom, especially formal education in K-12, for a number of reasons. Not all factors are included.
It can be difficult to accept authority. One underlying reason can be said to be authority is the spouting of errors that many people believe.
May engage in an intense form of mental concentration often described as a narrow focus resulting in a state of high productivity. Gets "in the zone". There are other “special states” that can be developed for access to specific unconscious processes, not just this particular type that tends to be more conscious. It can be something as simple as staring at a beach or trying to "taste the sounds".
There are more potential signs from pure observation from people who tend to be in a wide range of jobs that require a high level of cognitive flexibility and complex thinking from reading of many academic literatures. In some occupations people are classically associated with "intelligence" i.e., professors, there is a wide range.
Body signs are smart, some may have specific motor or vocal sounds, some facial expressions when thinking deeply, some rituals, talking to themselves (not referring). I also looked at other parts of the spectrum that are often not found in academic literature, people who look "dumb", "low conscience" or "miss the simple facts", and I saw some kind of genius to some who do not necessarily get along well with a “regular job” or “formal education”. It is quite difficult to know some forms of "hidden genius" and "hidden knowledge" that are often not easy for some people to express, but they can be clearly tested. Try to talk to commodity traders who by all means are statistically skilled over luck beyond reasonable doubt at audited returns, but far from the conventional image of a polished academic.
These are just a few things I can start with - this is not a complete list. Again, the brain is so complex, it’s really hard to cram a lot to make a short answer and you have to look at the big picture. People who put too much stock on IQ as a single intelligence number just lose.