Best as in: 1. Liquid 2. Minimum borrowing costs
The most obvious route is to short the lenders, preferably subprime. Since there are no lenders that operate exclusively in San Francisco, you could look north at Canada. The Canadian real estate market (esp. Vancouver) is just as overheated as the San Francisco market. As a start, famous short seller Marc Cohodes recommends HCG (Home Capital Group) as an opportune short.
You could short home builders who do a lot of their business in Northern California. (Not just San Francisco, Silicon Valley, or even the Bay Area.) Home prices in Sacramento and the northern San Joaquin Valley are correlated with Bay Area home prices.
Many of these builders went broke during the last bust, so you might have trouble finding a publicly traded home builder that is concentrated in just one market.