If you recently created your board you will have the new UI.
Older boards that were created with the old UI will have that with the option of adding the new.
To tell which you have, go to your boards Control Panel by clicking on the Admin link in your top navigation bar. Look in the sidebar of the page that opens for the DESIGN area. You will see one of the 3 representations below.
This is an old board with only the old UI:
This is an old board with the new UI added:
This is a new board with the new UI only: