Here's an easy way to check:
1). Type this, and exactly this into your browser bar: http://us.battle.net
2). Log in using your usual UN/PW/Auth Code.
3). Go to "World of Warcraft".
4). Go to "Accounts".
If you have indeed received a Beta Invite, there will be a new WoW Account listed, under "Beta". There will also be a download link.
Method #2:
1). Go to www.worldofwarcraft.com.
2). Go to the Forums.
3). Go to the Beta Forums.
4). Log On with your usual BNET UN/PW/Auth if needed.
If you can post in there, you were invited to Beta. Go to your BNET account and download your Beta client!
If you cannot post in the forums and if you do not have a Beta WoW Account, you were NOT invited yet.
Do not open that link, even if it appears legit.
Edit: BTW the REAL email (I received one) has lots of images (it is not plain text) and has no links at all, except to the forums, posting guidelines, and the forum Code of Conduct.
There is no, I repeat, no "Log In Here" or "Go Here to Install" links. None in the official email.
Edit2: There are also no "Beta Keys" or anything like that. If you were invited to the Beta, a new Beta Account is automatically added to your BNET account system and nothing is required on your part to "activate" the Beta. All you need to do is log onto your BNET account and follow the instructions on how to download and install the Beta client.
Edited, Apr 11th 2012 12:21am by Lyrailis