Mostly, by knowing how Internet addresses work. You read them from the right-hand end. The right-most part tells you what kind of computer system the user is on. Then you get an abbreviation for the organisation name, then maybe a department name or the name of a computer. Instead, they say what kind of site they are.
Addresses ending with .mil are definitely tops for prestige.
This is not because netizens have any particular feelings about the US military. Gurus On Duty have military addresses. Here, user-names are important. Telling these apart depends on experience and prejudice, particularly when reading Usenet news-group discussions. AOL is notorious for posting each message several times.