The SRID for San Francisco with meters as the unit is 26910. Let's verify this on the spatial reference website (click on an image to enlarge it).
1. Go to the website, and click on the EPSG link.
2. Type the code you're looking for (26910) in the Search Reference box and click the Search button.
3. Click on the resulting link.
4. Click on Well Known Text as HTML.
5. On the new site, you can check basic information about your SRID. Be careful not to confuse the two entries that contain the word UNIT: one of them is given inside GEOGCS (aka Geographic Coordinate System) and is not of much interest to us, while the other entry with the word UNIT outside GEOGCS is much more important and describes the unit returned by PostGIS functions. Note that you can also check the projection type here.