It sounds like you've already checked with the City as you refer to the Residential Record Report (3-R) common in San Francisco. Unfortunately, as you're finding, this report only tells you what permits may have been recorded, for what purpose and the dates for these. It doesn't, as you've probably found, tell you whether or not a "legal unit" was permitted by the City.
I have had numerous such situations over the years in both San Francisco and Daly City, a common ocurrence for many houses with "mother-in-law" units (as they are called). I have found in dealing with the municipalities in either cities, that there are some rules of thumb about whether or not something could be "warranted" or "acceptable" by the City, and it all has to do with "health and safety," alas, something that we all know had different standards in the 20's.
You'll probably find if you dig deep inought, e.g., go to the building records, look up building plans, if there is a permit referenced in the 3-R there'll be a plan filed along with that, that the "unit" is most likely NOT legal, particularly if the entire neighborhood is residentially inclined, or if you have a garage for only one car, or if your kitchenette is not properly ventilated, etc., in spite of the period decor or trim.
Our resourceful dads or grand-dads often took it upon themselves to build without the benefits of these now highly important permits.
Hope that helps.