intelligence is very contextual.
Emotional Intelligence , Logical intelligence , Common Sense , all are different kind of intelligence , but just like we cant judge a fish by its ability to climb a tree , its not possible to say if a person is OVERALL Intelligent (in all the ways)
So , it takes observation and experience to know if the person has the kind of intelligence that you are looking out for.
Best way , in my opinion is , to watch this person silently . And ask yourself , if you could come up with better solution in the given situation.
IF 7 out of 10 times , the person has better answers , than he or she is more intelligent than you , FOR THAT GIVEN SITUATION OR CONDITION
360 Happiness have mentioned some very valid points.
Based on life science, following signs are also personality of intelligent people, just in case if that helps, here are they -
1. **Quiet & Observer :-** As said, The loudest person in the room is rarely the smartest. They might claim to be intelligent, but that’s rarely the case. Sure, you might assume this person is an introvert or shy, but that doesn’t take away from their intellect. Quite People wish to think about what should be said before opening their mouths. The same cannot be said for those who are loud and brash.
2. **Handle Problems With Passion :-** An intelligent person refuses to back away from a current or pending problem. They face it head on, and that’s what separates them from everyone else.
3. **Enjoy Self-Criticism :-** Self-confidence is often confused with narcissism. A smart person is willing to accept criticism and often opens themselves to these situations as a way to improve who they are.
4. **Enjoy Multi-Tasking :-** Instead of focusing on one project, a smart person expands his/her list of things to do because it’s a battle against themselves. They have a thirst for knowledge and wish to push towards greater goals.
**Above points are just basic points that is usually found in Intelligent people, But To find How Intelligent they are? You need to follow what 30 Happiness has mentioned in his reply**