Quoting RooksBailey, reply 2
Is there a way to see the radius of an outpost? When it is next to a city, it just sort of blends in with the AOE of the city.
Is your grid up? With the grid it's fairly easy to see if you are within the radius (2 squares from outpost, 3 if upgraded)
Options > Show grid
OR
CTRL-g (in-game)
Thanks! Wasn't aware of the grid option!