I just know? Ummm....
- they don't sound like me.
- They think and feel and say and want (and do) things that are definitely not things I think or feel or say or want (or do).
- they tell me they are definitely not me
- they know things I do not know (like things that can be confirmed by others)
- they have different handwriting
- they like different things to me
- they can be completely different to me (in mannerisms, affect etc)
- I know I am not them!

Stuff like that.