It has been my experience that the airline is required to check your passport wherever you board your first flight on an international itinerary, even though you are flying a domestic segment to get to the hub from where the international segment departs. Where I checked in on line and didn't have to go to the counter to check a bag but went straight through TSA to the gate, Delta paged me and when I got to the podium at the gate, they asked to see my passport.