This is frequently done on (pseudonymous) imageboards, as well as chatrooms and the like. The admins have a password/key that when provided, enables them to be

This is frequently done on (pseudonymous) imageboards, as well as chatrooms and the like. The admins have a password/key that when provided, enables them to be displayed differently, like an additional image tag, or a separate nearby text element saying [ADMIN]. This is coded into the server-side site code, and normal users are not able to inject content into that other element to masquerade as admin.