By Matthew L. SchoonoverArbiter Series, Vol. 1
Agustus Pilot, a detective assigned to Special Services, is not a new kid on the block. in fact, he’s an Arbiter, a supernatural being assigned to solve any paradox between God and Satan for the disposition of contested souls. He’s not your average cop. These aren’t your average criminals.
Answer: According to Stephen King, NONE! In his excellent book On Writing, he said he just sits down and writes his story. If there's something he can't envision he just makes a note in brackets
[ find out what a Stutz Bearcat looks like]. Then once the first draft is COMPLETE, he goes back searches for the brackets and hits the Internet for a picture so he can insert a description...
Of course handling the past IS a bit trickier than a contemporary story, trust me I know. Most of the background for my Spirit Series came out of tales my Great-Granny told me when I was small. She had a hilarious one about trying to climb aboard a streetcar wearing a "Hobble Skirt." I'd sure have to look up a picture of a hobble skirt! She said you couldn't step up without showing your whole leg up past your knee!
You can use a slang dictionary to give the language a feel for the past, but be sure not to have them say "Twenty-three Skiddoo!" every other paragraph. Pick one or two phrases from the period, but be sure not to overdo it. With slang in any book it's always better to make something up, than to use stuff that will inevitably sound like a cliche. The trick is to not use anything that would not have been around then. Harder to find than hen's teeth, would be fine then, but NOT harder to find that moon rocks. And pick something that will be easily understandable, now. Even in this day and age, where chicken comes from the supermarket, anyone who ever saw Foghorn Leghorn knows that chickens have no teeth.
Then, I'd suggest you use "automobile" instead of "car," Or even better, be more specific, say a Model T roadster. If you need an expensive or classy car, make it a Packard, Pierce Arrow, or La Salle, because those were luxury cars then, and Cadillac is still around now.
Use some brief images to show objects that will help reinforce your time period, as well. Have the lady of the house straighten a pile of Coronet or Liberty magazines, or wear a cloche hat, or some other dated item, but NOT a hobble skirt. They were before WWI and far too early. Now you don't have to do this a lot. More than once or twice a chapter would be overkill. Just give one essential detail now and then and it will remind the reader of the era and give your story just the right touch.
Also Time Magazine put out a great series of books (I think they were called Time Capsules) with things that happened in specific years, lots of photos in them, so if you can get hold of the exact year of your story, that would be all you'd need.
Just don't forget our old friend Steven's advice. ALWAYS TELL THE STORY FIRST! Then worry about the research.