I am not sure to much about how to tell if someone is DID but here are a couple of links that have info on DID. They are posts that are sticky in the DID forums. I hope these help.

UNDERSTANDING THE DISSOCIATIVE DISORDERS
http://forums.psychcentral.com/showthread.php?t=47621

Dissociation vs. Dissociative Disorders
http://forums.psychcentral.com/showthread.php?t=89088

I hope these links help. As for what you can do as a friend. Do just that be a friend. Dont keep asking who she is if she is DID. Dont run away. Be there for her and listen and be understanding. For her this may be very scary. It may not seem real. Just validate her feelings. Just being a friend is a big thing.

Diana