How can I find a stud from the exterior to mount service mast?
Plan A


Find an interior:exterior reference point... a window, electrical penetration, etc. on the wall in question. 
Using a studfinder, or by examining baseboard nails and outlet locations, find a stud from the interior. You can often knock with your knuckles and determine to which side of an outlet the stud lies. 
Measure from the stud to your reference point. If it's a window, measure to clear glass.
Outside, measure back from the same point. 


Plan B


Gently release the bottom edge of one row of siding from its retention rib near the intended mount location. 
Begin drilling small holes behind the loose siding at 1" intervals, just deep enough to reach framing. 
Once you hit a stud, drill 1/2" left and right to find the rough center. 
Take a measurement from a fixed point, such as the siding corner or a window.
Flex the siding back onto the retention rib. 


No "exploratory holes" through your siding or wounds in your drywall are needed whatsoever here.