While flying having trouble estimating how far I am from Airport
My GPS tells me the distance. 

Otherwise, my experience says:


15 miles (and about 3,000 AGL) is where I'm not entirely sure I have the airport picked out.
10 miles is where I can see the airport, but not pick out specific details and airplanes.
5 miles is where I can see the airport clearly, and setup for the centerline.