I would say there are 3 major tells for this:
1. Precision of thought.
2. Listening. Not necessarily to other people but to whatever they are talking about. Listening to the subject and not enforcing their will on it.
3. Delicateness of thought.

Not all three tells need to be there, but at least 2 out of 3, especially as they are hard to detect.

1. Precision is not always easy to see as often it is hard to express oneself well. It's easier to speak clearly when you don't care about precision. Like that you don't need to speak the truth but what is close enough to be easily described. As description always requires some such sacrifices of details for clarity it isn't always easy to see when the person is being precise.

2. This one is the easiest to see. People who are vain rarely really listen to what they are talking about. This is why vain people definitely (sadly) gain short term success but rarely greatness.  In fact, never greatness, as arrogance  may accompany greatness but not vanity. Without listening you just throw your own theories where they don't fit.

3. Some thinkers, artists, people, have delicateness of thought/action.  Many great thinkers don't have it, but when it's there, the person is never vain.

How about if “people who show self-confidence, that is a result of their knowledge and assurance in their field” were driven by vanity too? Salomon came to this conclusion after he built his temple, vanquished armies and conquered the queen of Sheba. “What profit has the worker in the work he does”? Pain, disappointment and sorrow that comes with wisdom. Why would he do it then if not driven by the illusion of eternity, which is vanity at its core?

Hicham.

Hi Michel, I liked your answer and especially recognized the truth in what you said in your second point (looking back at some cases of people who led me on with their vanity- they all fail on that point) : "2. Listening. Not necessarily to other people but to whatever they are talking about. Listening to the subject and not enforcing their will on it."
I agree this point is the easiest to see, but I didn't really understand what you said this is a way for them to gain greatness. Do you mean that by constantly talking and forcing their will on reality, they convince themselves and others of what they wish to be?
My next question is how would you describe delicateness of though/action? Is it about the ability to have  wider consideration of things, or is it something else?