Legally, the airports location is the "geographic center" of the airport. That is what the 5 miles is based on.
You can google the airport and get its lat/long, plot it on a sectional chart, plot the location you are measuring to, and measure the mileage using the latitude hacks on the chart.
More simply, you can plot your position and measure to the center of the airport symbol on the chart.
Interesting... If I map the edge of my flight area and measure the distance to the SIDA fence, it says I am 5.1 miles away. The center of the airport would be more than that distance. AirMap says I am well within the the five mile distance.