I noted that I was an introvert a long time ago. The tricky part is: it’s not extreme. I enjoy socializing quite a bit. But, who doesn’t :). I suppose a real introvert would not. The point at which this became clear, was during my stage fright. I adore singing and was in choirs for many years & from a young age, but something frightening happened when I performed solos. I felt safe when performing around other singers. But when singled out: it was just me (my voice), the stage, a microphone and the audience, everything changed and became much scarier. All the attention then was on me. Something I dreaded.
I find proof of this in my career also. I’m a free-lance writer. As if just being a writer wasn’t solitary enough, I chose to be self employed.