Originally Posted by
Antarius
on the OUTBOUND, yes. For example SFO-LAX-LHR, they will check your passport at SFO at check in.
The OP is referring to the inbound. If you fly back LHR-LAX-SFO, they do not (typically) check your passport after arriving in LAX (after immigration, of course) for the LAX -SFO flight.
They don't, nor should they. But I've seen a lot of people chime in with "well, they checked mine on the outbound!"