I think the real test is time and seeing the person in various situations.
If they are putting up a front:
- They tend to need to always be in control in some way and when things turn reciprocal or require some vulnerability, they avoid it (will do favours for you, but won't let you, or only when they decide when and how, don't reciprocate the same level of interest in the things/people that matter to you as you do for them, share their knowledge or expertise liberally, but never require yours etc).
- They do a bit of push/pull so that when they start feeling dependent on you or realize they are getting too attached, they distance themselves for a bit.
- They avoid expressing gratitude or need for anyone else. Their success is due to their own good management and expertise.
- They seem very natural and comfortable in familiar surroundings where they are in control, but when put in a situation where they are out of their element, they may act rude, inconsiderate of you, be purposefully argumentative or impolite to the people that matter most to you etc.
- They may feel threatened by anything or anyone that matters a lot to you and devalue it subtly or not so subtly by their comments, choices or advice.
- They downplay your achievements or your interests in comparison to theirs so that they feel competent or avoid any activities which could showcase it. This happens especially if they feel that you have expertise that they do not.
- They act inconsistently, depending on whom they are around (their image matters and they want the best of all worlds). Often they will show the most value for the people they respect least or who are of the least lasting significance to their lives (easier to maintain a facade and also not be called on certain behaviours or choices. Also, the opinion of acquaintances and strangers is more important than the regard of those close).
- Careless about maintaining relationships in their lives, even with those who try hard to. Regular changes in friends, lovers, or employment is often an indicator of this. Maintaining relationships of any kind over a long period of time requires honesty, vulnerability, expressing needs, and conflict resolution. This is very difficult for someone who is not confident.
- Critical of others. At first it may seem flattering that this smart and discriminating person chooses to spend time with you, but over time you realize that it is a way of backing into attachment with another person in a less vulnerable way (bonding over what you dislike instead of what you like), and also attempting to feel superior to people that they feel threatened by).
- Sense of competitiveness. When you have a strong sense of who you are, you can afford to acknowledge other people's good qualities. When you do not, the only way to feel better about yourself is through proving that you are more competent, that you can win more, that you are more successful, richer etc. There is a need to borrow identity from other people or external accomplishments instead it being intrinsically felt. This is what little kids do when they first meet each other. Since they do not have a strong sense of what they can do and what they are all about, they need to prove their worth through the accomplishments of those close to them (my dad's stronger than yours!), their strength, their rank, what they own, etc. Some people don't outgrow this.
- Disinterest in service unless there is some benefit to them (improves their image, advances them in certain people's opinions, etc).
- Adopts behaviours or values when they meet someone that they normally wouldn't pursue alone or in other company.
- May use secret coping mechanisms in private to deal with inner anxieties or discomfort (drinking, drugs, etc). It's often the strongest appearing people who are actually the most fragile or sensitive inside.
- Often are the ones who do the rejecting at the end of the relationship (or just act poorly enough that the other person breaks it off) rather than discussing it openly. This is one way of pushing people away before they can get too close or can reject you.
- May seem interesting and complex at first, but often that "complexity" is due to their inconsistent behaviour (caused by underconfidence).
I wish I had been able to recognize some of these signs much earliert in my life. Most of the people I know who are like this are smart, interesting, competent people who have some very appealing qualities. However, this kind of insecurity can become very alienating, controlling, unhealthy or distressing to deal with on a regular basis. It is impossible to get close to someone who is underconfident and cannot trust you enough to be vulnerable and honest with you. I don't think it is a matter of rejecting someone as a friend or partner because they are underconfident, or not good enough, so much as that they are not going to be invested in the relationship until they have their own needs met and therefore have more realistic expectations of you and attention left over to devote to their intereactions with you.