Open WhatsApp
Go to the Chats tab
Tap the search bar at the top
Type the name of the broadcast list
If the broadcast list appears in results, tap it to open
If it does not appear, open the chat with one of the recipients and check the broadcast list there
On iPhone, swipe down on the Chats screen to reveal the search bar if needed
On Android, use the search icon at the top if the search bar is not visible