Imposter syndrome is a feeling you have, that runs counter to a rational evaluation.
You can see that you are objectively doing good work, and others will tell you the same, but on the inside you feel that you just got lucky, that you are just scraping by, that the others are just being nice.
You might also constantly compare yourself to an unreasonably high standard, internal or external, that you feel you are nowhere close to, and never will be.

Being self-aware enough to notice your thoughts without instantly believing them is key here. For some people, that comes naturally. It might be connected to your home environment. Did your parents model questioning their first reactions? Did they say things like “I keep thinking I left the oven on, but that’s just my anxiety talking”? Or maybe they helped you dispute your own beliefs by leading you through the process: “I know it feels like your friends are excluding you, but can you find evidence to the contrary? Didn’t they work very hard to throw you an awesome party just last week? Why do you think they did that?”

Another way to learn these techniques is to find a good therapist. A professional should be able to quickly suss out any unhealthy thinking patterns. Often, the first course of action is just to notice these recurring thoughts and maybe make a little mark on a piece of paper whenever they come up. Over time, you can replace them with healthier beliefs.

Somebody in that process would know that they have impostor syndrome because they’re aware of thoughts like “I am such a failure”. However, because the thoughts still occur and need to be actively disputed, this same person does still suffer from the accompanying low self-esteem.

I think many (most?) of the pictures posted on FB and Inst are from people with imposter syndrome. They post a posed picture that is only a fraction of a second of the true story. The instant smile hides that they had been fighting all day, don’t have the money to pay for those drinks, were intoxicated and throwing up a few hours earlier, etc.
If they were really having such a good time they wouldn’t be on their phones posting pictures.

The impostor syndrome is the belief that you are not really as good at something as you are objectively perceived by the outward world. You are actually performing very well at whatever it is but your low self-esteem makes you think that you are faking it. The cognitive dissonance between the affirmation you are getting from others and your own perception of your inadequacy makes you feel like you are an impostor.

I guess your question is how do you know that your perception is a false one? I think it is through experience – seeing again and again that you are actually performing well in the eyes of others so that you begin to realize that your own self-image must be false.

Great question!

Imposter Syndrome is my middle name.

My definition of imposter syndrome: Picture a social media consultant, and think of the perfect opposite of that.