It's like smut; you know it when you see it.
There's the paperwork side of it- got a driver's license that has Walton/Bay/Okaloosa/Holmes/Santa Rosa on it or a home with a homestead exemption on it? You're pretty much a local. Got a Sunpass or an Eglin rec permit? You're getting close.
Part of it is experiences- You've learned how to either evacuate or hunker down, and have learned the sometimes subtle difference between the storms you buy bottled water for and the storms you buy beer for. You've lovingly mulched and wrapped the tender perennials in your garden when we've gotten one of those January cold snaps where the thermometer hits 28F. You've gone to high school football games, and have an opinion on the Freeport v. South Walton Big Game. And you've watched the traffic heading north on the bridge on Labor Day and felt a vague sense of relief that the traffic and congestion was going to settle down to manageable levels until spring break.
Part of it is how you interact with the community- A job gets you a lot of points there. Volunteer work gets you a lot of points too. The snowbirds who come down and volunteer to tutor in local elementary schools get honorary local status. Same goes for the people who work coastal clean up day, or work water stops for the charity runs. Also bonus points for joining a church or civic group, or selling/buying/donating something to sell for local school or charity fundraisers.