Oh! Sorry for misunderstanding.
All I can say is that I just "know" when it is an alter because I absolutely "know" it isn't me. I just "know" it is an alter and not me. I don't know how to explain it other than that. They are just not me. (Although of course I know that in theory that they are a part of me. It's just that my experience of them, their thoughts an feelings is as an "other", a "not-me".)