On your list of games where it says World of Warcraft Cataclysm [Account name], etc. it would say World of Warcraft Mists of Pandaria Beta and then for the realm it should say Public Test Realm, or something similar. Just on the "Your game accounts" in the summary list.
From what I hear, it'll be very obvious. On your account page, it tells you what in battle.net you have an account in (as in, which games). People say it should say "World of Warcraft Beta" under the World of Warcraft section.
Don't worry, it won't be something hidden, or something you have to look for.