The easiest way is via your Inbox. At the top of the left-hand most Inbox column, you will see a row of icons.
Click on the Calendar (just to the right of the star “Favorites” icon). Then choose ‘Single’ or ‘Group’ SMS.
The system will return all scheduled messages in the category you selected.
Alternative: If it is a group message and you know the group name, you can go into Groups, select your particular group, and you will see all the scheduled messages for the group you selected.