It will often be assumed their name is a nickname. If your name is Jim, people are all going to assume it's short for James, and you'll always have to correct them.
That's interesting because where I'm from it's the exact opposite. If someone is called
Jim no one would think it wasn't his full name. People would assume it was his full name and they would probably be
very surprised if it turned out that this
Jim's full name was
James. I guess the full names and their nicknames have been "lost in translation". I'm sure there are many, many that don't realise that names like
Jack and
Bill were originally nicknames.