Locals refer to themselves as Washingtonians. We don’t say D.C. At most we’ll refer to the city as “The District.” I really hate this new “DMV” trend. That’s the Dept. of Motor Vehicles, not D.C.-MD-VA.
Suburbanites will say D.C., usually in disdain. Newbies say D.C. for a short time. But, a true Washingtonian says just that. It’s much like San Francisco natives never say “Frisco.” They’re San Franciscans.
So make a friend and ask if they’re a Washingtonian. Sounds much smarter.