So, there are couple of ways to do it. Sometimes when I'm playing a name will come to me and I'll think "Oh, is that available?" I try to add the name as a friend with "/friend <cool name in question>" If it adds to my server friend list, then the name is taken. If it says they aren't available or something then the name is free.
Otherwise, you have to be in the character creator, but at the first screen you can type the name and select the search glass next to it and it will validate if the name if available or not and if so, it will reserve it while you finish creating your character.