Two or three things happen when a GM wants to talk to you, as mentioned above.

1, you are prompted with an on-screen visual cue that informs you of a chat request from the GM. I forget the exact wording of the message, but I'm fairly certain there's small popup window that appears and informs that "A GM would like to talk to you", or somesuch. Also, if you have an open ticket, the small tooltip that says "You have an open ticket." next to the minimap will glow a cheerful golden color. You can click on it to accept the chat request.

2, upon accepting a chat request from a GM, a second chat window will open. If your chat box is in the default position, the GM chat box will appear right above it.

It's also important to note that Blizzard employees will ALWAYS have the happy blue BLIZZ graphic before their name. As stated above, some player may have accidentally gotten one, but a genuine GM will always have one. If there's not a happy blue BLIZZ icon, it's not a GM.

Let me also add that Blizzard has the best tech support in the world. They're very courteous, friendly, and professional. I've never had I conversation with a GM that I didn't like.