Is it possible to check-in and board from the stopover point of a 2 (or more) segment flight? What if it is an international flight (online checkins not allowed since you must present your passport)? This is assuming that I have no luggage to check.

The (ua) LAX->SFO->PEK flight is less expensive than the SFO->PEK flight, but I would like to simply check-in and board from SFO.

Thanks.