If you click on someones profile you'll see a popup of their profile, at the bottom are a selection of the badges they've earned and the first is the last level upgrade.
Some categories have a lock symbol on them, that means you can't view them without creating a log in. If you send someone a link to a topic in one of those categories then they will have to at least log in. For the most part these are the Health categories, and the reason they require authentication is primarily to prevent them being indexed by google.
If you send them a link to a topic in a category without a lock anyone can view it without authentication.