Originally Posted by reimero

They'll check your documents at LAS, and I do recommend showing up there somewhat early, because a human being will need to see your passport. That said, it'll be fine if you check in online and then use a mobile boarding pass to get through security; you'll just get paged at the gate to present your passport to the GA.
But basically, once you're checked in initially, you should be good to go: as soon as you're known to have shown up on time and with valid documentation at LAS, it's on Delta to get you to your destination.