Assuming it is purchased on a single booking you should be fine. If things go pear-shaped out of SFO it is on UA to make you whole (I'm assuming something like SFO-(UA)-SEA-(DL)-ANC as the itinerary).

If I have checked bags on an interline itinerary I like to check in with the gate agent (or transfer desk, if such exists) to make sure the second carrier shows that the bags are associated with the itinerary and in the system. And usually they want to print a "home" boarding pass. For DL/UA don't expect the second BP until you get to the connection point.