According to Genius Types, most insecurity springs from a lack of self-worth. If a woman feels she is not good enough at her job, relationships, intelligence or appearance, she will be insecure. She will probably express these doubts to close friends or family, so look for a high occurrence of worry about failure. The Weightless blog says women will seek excessive validation on her weight and other arenas. Try to reassure her that she is measuring up in these key aspects of her life, but be sure and tell her that she does not need external approval to feel confident.
Health and Nutrition says that jealousy stems from an insecure outlook, and fear that her partner will leave her. In a relationship, a woman might feel that she is not good enough to keep her partner from straying to someone else. This will manifest itself in increased arguments over where the partner is and how he spends his time, a desire to read the partner's email or texts and accusations of unfaithfulness or not loving her enough. Again, try to reassure her that she is worthy of love and fidelity, and have her consult a therapist if the problem worsens.
As Genius Types says, "Insecure people tend to be very sensitive to critique and respond with defensiveness." Because an insecure person believes criticism of her to be true, they react very strongly and can be aggressive toward the criticizer. This is also a part of being overly sensitive, which is another sign of insecurity. Watch for overreactions to small criticisms.
Because an insecure person often feels she isn't worthy of close personal relationships, she might express this feeling in terms of being lonely. She might feel like an outsider or as though people don't like her, and retreat into herself instead of participating in social interactions. If you notice your partner or friend retreating in this way, try to draw her out and show her that she is liked and has people in her life who want to be around her.
An insecure woman might have an issue of apologizing all the time, since she feels she is always doing something wrong. This can also show itself in her making a lot of excuses for her behavior, even if it is normal to everyone else. Insecure people also have a hard time saying "no" to others, which can make them stress if they take too much on, leading to more unnecessary apologies. Keep an eye out for this and let her know that people will not judge her if she can't do something or doesn't want to.