Cyan Update app will tell you when you can download WP8.1

This is the QR code to an app called Cyan Update. Install it and it will regularly check and tell you if your Nokia Lumia phone can be updated to the new Cyan firmware and Windows Phone 8.1 bundle.

It'l tell you on a live tile:

Of course, you could always check yourself by hitting Settings > Phone Update. But where's the fun in that.