We are approaching the end of San Francisco today. So, if you haven't started indexing, and you want to help with San Joaquin County, you have to jump in now. I would be willing to bet that it will become available tonight or tomorrow and be done well before next Monday rolls around. It might not even last more than a day or two.
The following states are already indexed and searchable by name at Familysearch.org:
Colorado, Delaware, Kansas, New Hampshire, Oregon, and Virginia
Another twelve states are fully indexed and just have to complete arbitration and the computer work to put the index online.
For all other states, browsing is fairly easy. Census takers had modern handwriting that are usually easy to read. You can scan a page very quickly looking for the names you want.
At FamilySearch.org or Archives.gov or Ancestry.com you can choose a state, county, and location and browse the enumeration district images from there.
Familysearch and Ancestry describe each enumeration district to help you choose.
Archives.gov has county maps from 1940 with enumeration districts (poorly) outlined as another way to find the ED you need. You will find a link to the maps after you choose state and county from the drop-down menus. You can look at the maps or download them. It is fun to see a 1940 map in itself, whether or not you are using it for the census information.
An alternate route is Steve Morse's 1940 Census In-One-Step that helps you find the right enumeration district using street addresses or 1930 enumeration districts as a guide: http://stevemorse.org/census/unified.html
From Steve's site you can link right into the viewers for any of the sites that have the 1940 census available for free.
Find out who talked to the census taker, who completed high school or college, and how much money they made in 1939. (A 1940 dollar is worth about $16.50 today. http://www.usinflationcalculator.com/ or http://www.minneapolisfed.org/ )