1. Seals on the retail box have been removed and box has been opened.
2. Mac boots into an existing user account or login screen, which indicates initial setup has happened.
3. It's dirty, scuffed up, or has nicks around the bottom case where a pry tool has been used.
Do you mean like used, then put back in the box and then sold to you? Or do you mean did someone walk up to it at your desk and use it?
If the former, the box is sealed and the computer comes with a sticky wrap around the ports that's essentially impossible to reinstall perfectly.