When you check in, if you haven't cleared, you should get a second page which is a Departure Management Card which shows that you're on the waitlist for the upgrade. If you don't get that, make sure you ask a ticket or gate agent to place you on the list (confirm they know it'd be complimentary based on fare class purchased).

Before flight, call reservations and again confirm it'd be complimentary and ask them to put you on the list.