Most of the people we talk to through text (like texts or online messaging) can tell who they're talking to by how everything is typed out and what kind of phrases are used, and whoever we're talking to will usually ask just to confirm who they're talking to. Honestly we don't really care if people have to ask, because not knowing who you're talking to would make keeping conversations straight easier. We have a tendency to switch in the middle of conversations and I can imagine it would be very confusing without any context.