The Lady From Shanghai - "You're The Killer, Yes!"
Elsa spots O'Hara inside the Mandarin theater and calls Li, her servant, from backstage. The call is routed through the Chinese Telephone Exchange to his room.
Then ... Li may be a humble servant but his place has a spectacular view of Telegraph Hill and the waterfront.
... and Now, this is the same view - it was taken from the tower of the Fairmont Hotel, so we conclude that Li's room was a studio set using, for the window view, a photograph taken from the Fairmont Hotel on Nob Hill (map). Columbus Avenue bisects the picture at an angle from left to right - the buildings below it have seen many changes, those above it, few.
Then ... When Li exits his house with two cronies the studio setting is confirmed ...
... on the back lot ... because this, as were several other scenes in the movie, was filmed at the Columbia Ranch back lot in Burbank, Southern California, on Brownstone Street near Skid Row. (see here for a map of the ranch). Below is a shot from a 1949 Batman and Robin TV show, filmed at Columbia Ranch, showing the same houses. The back lot is now called Warner Ranch and is still operational but these particular houses and streets were destroyed by fire years ago.
Then ... Director Welles adds more trickery when he cuts to the police heading towards the theater.
... and Now, why trickery? Because this isn't San Francisco's Chinatown - it's Los Angeles' Chinatown! (CitySleuth thanks reader Opium Museum who brought this to his attention). It looks south down Sun Mun Way just inside the East Gate of Old Chinatown at 951 N. Broadway (map).
Below, Elsa joins O'Hara in the theater to find him struggling to stay conscious under the influence of the pills he downed at the courthouse. But he's aware enough to figure out that she was the one who killed Grisby, the murder he was being tried for. We see from her ice-cold reaction that he is right but he collapses and Elsa's Chinese mafia move in and cart him off.