I feel part of your dilemma is that you are defining the problem using two scenarios that will appear the same to you without previous experience with it.
Allow me to explain. Let’s suppose you have been assigned to do the task X and we want to determine how you should feel about it. In your dilemma you ponder how to differentiate between something being outside of your skill set or merely being new and you may tell yourself you cannot do it. Both of these are just different ways to react to the same situation if you have not recently tested yourself against doing task X. Have you done it before and recently (since we need to account for change in skills over time) you can objectively know if it is within your skill set or not. Otherwise, this is an unknown and only after truly tackling it can you objectively know if you can or cannot do task X.
Herein lies the role of anxiety, because in reality we cannot always try everything just to see what we can and cannot do because there are real world consequences to this that make it not worthwhile to test every aspect of your abilities. So when presented with a task, you have described two ways anxiety can filter out for you your comfort level:
1. Feeling anxiety because you have reason to believe it is outside your skill set based on previous experiences.
2. Feeling anxiety because you have not attempted it before, so whether you can or cannot do it is an unknown (which is a hook Imposter Syndrome can use to get itself reeling).
These are not the only ways of anxiety can have a role in this scenario, but it outlines where the anxiety can come from. For your question about a mental model, differentiating between the two could by asking yourself the question, “What about task X makes me feel anxious?” If part of task X includes some aspect you failed at before this falls closer to what you call “real” anxiety and if it is just the fear of the unknown it is closer to what you are calling “fake” anxiety (to be clear, as like others commenting here I do not agree with these terms as all anxiety is a real emotion you are feeling). However, in the case of the latter it being an unknown still means it may or may not be outside your skill set, so as mentioned above you cannot objectively know this until after trying it. Short of this, that is where your judgement skills and honesty towards yourself would play a factor in how comfortable you feel on taking on task X given unknowns.
Edit: formatting
Allow me to explain. Let’s suppose you have been assigned to do the task X and we want to determine how you should feel about it. In your dilemma you ponder how to differentiate between something being outside of your skill set or merely being new and you may tell yourself you cannot do it. Both of these are just different ways to react to the same situation if you have not recently tested yourself against doing task X. Have you done it before and recently (since we need to account for change in skills over time) you can objectively know if it is within your skill set or not. Otherwise, this is an unknown and only after truly tackling it can you objectively know if you can or cannot do task X.
Herein lies the role of anxiety, because in reality we cannot always try everything just to see what we can and cannot do because there are real world consequences to this that make it not worthwhile to test every aspect of your abilities. So when presented with a task, you have described two ways anxiety can filter out for you your comfort level: 1. Feeling anxiety because you have reason to believe it is outside your skill set based on previous experiences.
2. Feeling anxiety because you have not attempted it before, so whether you can or cannot do it is an unknown (which is a hook Imposter Syndrome can use to get itself reeling).
These are not the only ways of anxiety can have a role in this scenario, but it outlines where the anxiety can come from. For your question about a mental model, differentiating between the two could by asking yourself the question, “What about task X makes me feel anxious?” If part of task X includes some aspect you failed at before this falls closer to what you call “real” anxiety and if it is just the fear of the unknown it is closer to what you are calling “fake” anxiety (to be clear, as like others commenting here I do not agree with these terms as all anxiety is a real emotion you are feeling). However, in the case of the latter it being an unknown still means it may or may not be outside your skill set, so as mentioned above you cannot objectively know this until after trying it. Short of this, that is where your judgement skills and honesty towards yourself would play a factor in how comfortable you feel on taking on task X given unknowns. Edit: formatting