I do not know why "would be" is needed at all, it sounds very odd to my ears.
If it IS Julie Roberts, then you just say "That is ..." and if you are not sure if a woman (whom you can see) is Julia, then you say "That could be" or "might be"
in your second example you need to say "It will be John" so I don't agree with Elroy - perhaps it is a way that Americans express themselves but it sounds not quite right to me in the UK.