if by 'base' you mean the airfield, it is seldom the focus of a GV attack. it is normally the town.
as a dedicated GVer, I've found the best defence is, like wastin says above, drive out and check the town, (if there's nobody in planes to do a flyby) then work back to the spawn, because that's where they're coming from and where they're easier to find, to find out where they're going. if an attacking GVer is smart and knows there's any defense up, he doesn't make a beeline to his target. beside, the auto-ack does a pretty good job of initial defense on GVs. I'd wanna start my defense as close to the spawn as I can.