Look at the Aircraft range on the air group detail screen. Divide the range by 10 (range is in miles) to get the number of hexes the plane can fly. Divide that in half to get effective range. For example, an aircraft has a range of 500 miles - that is 50 hexes which goes down to an effective range of 25 hexes from the airbase (25 out and 25 back for the total of 50).