Right at check-in, when you hand in your disembarkation form with all the
informations, is when you accomplish your goal. The ship staff, seeing you're
on an early flight, will '' tag '' you for first-off disembarkation ( you AND your luggage ).
If you buy their airport transfer, even better as your bag(s) will be wisked right to
the airport ( in most ports cases....)
Midwau thru the cruise, go to the purser and confirm you're indeed among the
'' first off '' passengers.