Sometimes I don't. When I change back to my child self I can completely forget that I have DID. I was hospitalized for "depression" two years ago and I didn't even mention to the doctors that I had DID- I had totally "forgot."
Usually when I'm saner I can definitely feel that I have more than one personality. One way is the obvious- that I act differently and have different values etc. Sometime though I can feel two emotions at once, like the child part of me gets sad and I can hear her cry in my head, but at the same time I'M not sad. It's ridiculously hard to explain.