It is very easy to known whether a person is insecure or not. Just try them on these important points....
Running down others' looks: Someone who is insecure about their looks will often point to someone across the room, or across the gym, or on their phone screen, and point just out how ugly they think they are, or what all their faults are, or how awful their clothes are. This is because they themselves are insecure. People who are okay with themselves never have a need to point out the physical faults or fashion transgressions of others.
Humiliating those they were once close to: Someone who is insecure about how they've acted or treated someone else will publicly defame or bad-mouth that other person, and even worse, they will offer only one side of the story. Theirs. People who didn't have a major part in the demise of said friendship or relationship have no need to defame the person from their past.
Bragging around: Someone who is insecure about how much they have accomplished in their lives, or in their careers, or in their relationships, or in their goals, will brag about everything they have done or accomplished. This is not to be confused with confidence. People who are okay with where their life is at, where it's going, and what they've accomplished so far, have no need to brag.
Belittling others' success: A person who views another person's success as superior to their own, or who sees the public praise surrounding another person's success and is intimidated by it, will belittle and diminish that success in any way that they can. People who appreciate the success of others, and use it to inspire themselves and push themselves to greater things, have no need to ever belittle a successful person in their quest to feel better.
Giving excuses for trampling on others: Someone who knows that they are treating other people's time and relationships as disposable will always have a "really good excuse" as to why they always show up half an hour late, or why they don't treat their friends and family better, or why they talk bad behind your back. Their excuses are always somehow noble and if you question them, it is you who becomes the jerk.
Real bad losers: There is nothing this type of insecure person hates more than losing. At anything. And when they do lose, they will tell you all sorts of reasons why their loss isn't really a loss, or why it wasn't fair, or why the other person had an imbalanced advantage, or how they know that the other person was cheating.
Horrible winners: They are either preemptively striking your next win, or they are covering up their past losses, or they are attempting to squelch any doubt others obviously might now have about their abilities. Good winners have no need to do anything but put out their hand and say "good game" when they do win.
Mocking others in poverty: Someone who is insecure about their own financial security will constantly and sincerely ridicule those living in poverty or in lesser circumstances. People who don't define their worth by the dollar amount in their bank account have no need to make fun of those in lesser situations.
Blind to others' positivity: Someone who is intimidated by another's greatness, light, or achievement, will work seemingly endlessly to bring that other person down. They will ceaselessly search for and point out to others the negatives and the weaknesses within those who make them feel most unconfident, and they will not, under any circumstances ever acknowledge the goodness that also exists.
They will make idle threats: Sometimes, people who fear that their insecurities have been discovered, or at least are under suspicion, will often make idle threats if you challenge their insecure ways of "acting out." They will threaten your good name, your livelihood, and your relationships. They will threaten to do harm to your good reputation. Someone who is not feeling insecure, will never have a reason to threaten you to protect their own insecurities.
Pinning you eternally to your past mistakes: Some people feel so defined by their past mistakes and their past failures that they refuse to ever let you move past yours. They search for any moment in time where you were less than perfect or where you made a mistake, and they will attempt to make everyone label you by that mistake forever more. They will not see any of the great things you have been, done, or accomplished since.
Always the last word: Some insecure people love to engage others as a way of feeling validated, intelligent, and authoritative. They will not concede any point, no matter how silly their argument is, and they will not ever agree to disagree. They always have to have the last word, or else they feel inferior and insignificant.
Everything to them is a hidden jab: People who are insecure about their ability to hide their weaknesses will think that almost everything you or I do or say about much of anything is a direct jab aimed at them, specifically. They will think almost any discussion around them has ulterior motives, and they'll constantly shuffle the conversation back to them in an effort to dissect the motives of everyone involved.
BDST: 1559 HRS, Aug-10, 2014