I think, Google Street View is a great help! I am from Germany and was in San Francisco just once, what I remember most are the steep streets

I took a lot of pictures. Besides that, you could try to read travel diaries or listen to podcasts or radiostations to get an insight on traffic, festivities or the overall mood. Then, just write as you imagine it to be, as many of your reader might also not have been there. Maybe you could then, after writing, find beta readers who know the city, and they might have some small details to add to the descriptions.
I guess, a lot of San Francisco's "fame" comes from the imagination of people and TV series (you could watch them too).
So it might be really far fetched, but to get the overall feeling, any American bigger city would do. In addition, of course the plot of the novel has to be really good, so the city is just a nice background to it.