How can I tell I'm thin?

I hear I am from my friends, but they know I have self esteem issues so I doubt their sincerity at times. Most of the time people are like "you're not fat!" but they normally don't say I'm "thin" or "skinny", at least not as much as "not fat". I have no self confidence so I don't know. :/ please let me know. I'm 5'4" wear a size 2 in most stores but weigh 135-140 depending on the day and scale, I have a medium build (bone structure wise) but I just can't think of myself as thin because of my weight, despite wearing a small/size 2 in most stores.
How can I tell I'm thin?
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