1.) Go to the Testserver char-copy webpage of World of Warcraft, where you can create premades or where you can copy an existing character.
2.) Choose "Copy a character" (from an existing realm) and select any realm in the list.
3.) The URL should look now like this:
4.) The parameter "realmId" is your selected realm. Now type any number in there an volla - your "selected" realm have changed.
5.) Scroll down to the list. If you haven't got any character it will say:
You have no characters on <Servername>.
6.)Now test it with any numbers and if you got luck you see hidden servers in the error message. Like "You have no characters on Aimee's Development Server".