Edit: As of the 8.1 update this is no longer valid. I am leaving the old answer below for historic reference.
As of the 8.1 update an Action Center has been added. You can access it by swiping down from the top of the screen. Similar to the IOS, you will have notifications there as well as shortcuts for bluetooth, wifi, airplane mode, etc...
For general Toast Notifications, There is no queue. Applications, however, keep track of them. The list below assumes you have the corresponding app pinned to your home screen
- The SMS app will have a number on the tile indicating how many txt you have. You can also see this from your lock screen.
- Email doesn't give you a toast notification, but you can see how many you have on the live tile. This number isn't how many unread messages you have, but how many you have gotten since you last opened it. This is also on your lock screen.
- Rowi (Twiter client) has an option for toast notificaiton for direct messages, and mentioned. It shows how many of these you have on the tile as well. There are other applications that do this as well.
In short, there is no "unread notification queue" to check, but each app has its own experience. Try pinning the application to your home screen for quick visual feedback.