If someone calls out "is that you", and I don't know who they mean by 'you', I might say 'It's Hilary - who did you think it was"?. If I do know who they mean, then I would reply, 'no, it's not......., its Hilary'.

You could just say "No, that's not me." For example, someone points to a picture and says "Is that you?" If it's not then you could just respond "No, that's not me." If you know who it is you could say "That's Sophie" or "That's Natasha"