Q u o t e: When the Beta dose start how will i know if a invite is really or phishing?
We do not have a date announced for the Beta phase of Cataclysm yet, Jetreaper. When more information becomes available it will be posted in the General Discussion forum and/or the Cataclysm forum.
If you have opted in and you are accepted into a Beta Testing Phase you should receive an e-mail notifying you. The Beta should simply be added to your Battle.net account, allowing you to download the client.