Intelligence, in humans, can be very difficult to describe or even define, but one easy-to-understand definition is that it is the ability to apply knowledge to manipulate one's environment. It can also be described as the ability to reason and/or have abstract thought and to adapt to one's environment. No matter how it is defined, everyone agrees that sentient beings like humans exhibit certain signs of intelligence as they grow.
The Ability to Interact with One's Environment
One of the biggest indicators of intelligence is the ability for one to interact with the surrounding environment. This does not only refer to a physical environment. Interactions with the surrounding social environment can come into play as well. However, intelligent people will learn how to interact with a physical environment first. Humans learn through trial and error how to best benefit from their surroundings, and they will display problem solving and adaptability skills of considerable complexity.
Humans become self-aware at a young age. This can be considered the beginning of intellectual development. Being self-aware means that you are conscious of your own existence. Without self-awareness, people would be unable to know what emotions they are feeling, and they would be unable to express them in a healthy or "acceptable" manner. As people become increasingly self-aware, they learn how to identify their emotions and monitor their emotional responses.
Being able to follow one's intuition can also be a good indicator of intelligence. This is something that develops relatively later in life. Intuition is not formed in a vacuum, but from the combined experiences of a person's life. People learn through trial and error whether they realize it or not, and those unconsciously accumulated experiences can tell them what is a good idea and what should be avoided at all costs.
The ability to not simply react to a situation but to recognize and consider other options based on one's environment is a good indicator of intelligence. This is called flexible thinking, and it is the basis of creativity. The ability to use tools or recognize and use information that may escape some people are just some of the results of flexible thinking. The ability to create works of art and literature are perhaps the ultimate expression of flexible thinking.