Since census data informs one of family members and relationships - city directories are the next best thing - not including birth and death certificates of course (in my opinion). If Joseph Marks has the same address as Louis Marks in 1888 in San Francisco - I can surmise (although it is not proof) that Louis and Joseph are likely related. In this personal case, Louis was my great great grandfather, and his son Joseph my great grandfather. And there is a lot of other information that can be gathered from these old directories.
I became excited about San Francisco city directories when I had a Footnote (now Fold3) subscription many years ago. They had a nice collection of older San Francisco directories. At that time I could browse and search through the directories. Fold3 requires a subscription.
Then I moved to the Internet Archive and found many more in lots of different cities. Internet Archive has a nice page flipping feature that makes it pretty easy to look for specific surnames once you select a specific city directory publication - and it is free.
I also discovered Don's List - which also has a nice collection of directories and is free and browsable. There are also international directories available.
Ancestry.com has a large collection of directories that do have an index, so that a surname or compete name can be entered and one can find an ancestor in the results provided. The page where the person's name exists is made available and all one needs to do is scan through the image to find the subject person. Ancestry requires a subscription.
Distant Cousin also has a search and browse capability for city directories which is free.
Mocavo has a number of city directories of interest to me. Again, you enter the person's name and the results list shows the page where the subject "resides" and as a nice added feature - it highlights the person's name. And searching city directories is currently free. Although I have a Mocavo subscription, I have tried it with a "free" account and the results are the same.
England and Wales trade directories are available for free at the University of Leicester Historical Directories project. It has browsing and search capability.
And finally, Cyndi's List has a very long list of smaller collections of directories online provided by local libraries and historical societies. Definitely worth looking at these free resources.
So try out all of these options - there are plenty for you to choose from. And city directories are really fun to search. And rewarding.