You'll know if you get in through an Xbox Live message formally inviting you to the test ring.

This isn't for the faint of heart. Alphas are rough by their very nature, and Microsoft warned that the current Alpha Skip Ahead build has bugs that can prevent the home dashboard from loading or prompt you for a passcode even if the feature is turned off. You may only want to sign up if you're determined to try new features and feel that even beta tests are too slow.