According to this thread, EK should be able to tag your bag all the way to SAN. DXB First Class Check-in - Will they through-check bags to interline partner?

However, you still need to collect the bag in SFO, and recheck it with UA post-customs at baggage recheck. You should also confirm with a UA agent that the bag tag information is in your PNR and in their system. Might be easier, overall, to just check the bag with UA at their counter separately in SFO.