• "I need someone to remain in a coma for six weeks."  
• "What kind of internal injuries would be serious but generally not life-threatening?" 
• "I have an arsonist character that can't get caught. How can he avoid it?"
• "What kind of handgun would an old vet pass on to a son going to war?"
• "What color is the blood when someone's shot near the heart -- dark red or bright red?"

• No one drives on the lower deck of the Golden Gate Bridge because there's only one level of roadway. 
• Marin County isn't south of San Francisco.
• There's a hairpin turn off Presidio onto West Pacific, near the golf course.
• What were the dates of the Zodiac killer communications in early 1974?
• What's at the top of Nob Hill, and what's the view?

It helped to grow up near San Francisco. But if you use an unfamiliar locale as your setting, there are ways to get a sense of place -- photographs, interactive online maps that show actual street geography, reading local newspapers, public records, even a brief visit if your pocketbook will allow it. Notice these details. Pay close attention to color and sound and light, because they can bring your setting to life.  

I have to admit that it helps to have a book already under your belt to prove you're an author -- when you write mysteries and thrillers, asking questions that are related to crime and murder and arson can raise eyebrows, especially if you're not well known. 

Just be prepared for some semi-serious teasing about exactly why you want the information! 

Then go write it down.