Last time I flew from SFO to Canada on United (several years ago), check in was at United's domestic check-in desk.

Be forewarned that it took an hour to get to the front of the (non-elite) line, and on-line check in was not allowed for flights to Canada at that time. If you are unable to check in on-line, arrive early enough with the expectation of being in the check in line for an hour. This means arriving at the check in line at least one hour and 45 minutes ahead of flight time, since United requires you to check in at least 45 minutes before flight time for flights from SFO to Canada ( http://www.united.com/page/article/0,6722,52481,00.html ). Better to make it at least two hours plus to be safe, especially if you are not an experienced traveler or are unfamiliar with SFO.

If you can check in on-line, but have baggage to check, United's baggage drop line for those who have checked in on-line is probably 10 to 15 minutes.

Note that United and Air Canada code share a lot of flights. Make sure you check in with the airline operating the flight.