The very first time I became aware of my alters was when a group of them showed themselves to me. Like, the lliterally "showed" themselves to me. Internally, they stepped into the light as it were and I could "see" them. I saw their individuality, they told me their names, and I realized I "knew" some of them. I recognized one as the one who was always getting me into trouble and another as a feeling and voice that I had sometimes been aware of. So in one sense I had been aware of them for a long time, but in another I was only just getting to see them for the first time. But once they showed themselves to me there was no question in my mind that they were themselves and not me. Even in retrospect I was aware of them as "not-me". I was always baffled by the "not-me" troublemaker and the "not-me" voice, I just didn't have the awareness to know that they were alters.
I don't think that is helpful to you in any way at all, that was just me sharing my experience of it with you.
Sorry?