I find it's best just to call it "San Fran" to disambiguate.
And for the entertainment value of indignant inhabitants of the area gasping like goldfish out of water as they prepare to tell you that no no no, it's not 'San Fran' and only someone completely ignorant of the most revered status of their fair (or foggy) city would make such a horrible faux pas :-)
Not a problem, no one cares what you call SF any more. We'll just know that you're not from around here. And that's ok; we like meeting new people with strange and interesting ways of saying things.
And for the entertainment value of indignant inhabitants of the area gasping like goldfish out of water as they prepare to tell you that no no no, it's not 'San Fran' and only someone completely ignorant of the most revered status of their fair (or foggy) city would make such a horrible faux pas :-)
"Frisco" works, too.