I found some interesting images.
First from posterior view, displays the radial nerve path in non rotated arm.
From Lippincott Williams & Wilkins Atlas of Anatomy (Tank, Patrick W.; Gest, Thomas R)
Here is anterior view (front view).
The spiral groove is only area where nerve is located on humerus bone. There for fractures leads to damage of the nerve.
To find where the radial nerve begin we need to check muscles, especially teres minor and deltoid:
References: Great sites for anatomy study!
Virtual Medical Centre
Virtual Medical Centre.com - video: Shoulder joint (including humerus articulation/rotation)
Pediatric Orthopedics: Humerus bone - detailed views
Pediatric Orthopedics: Muscle anatomy of Arm
Lippincott Williams & Wilkins Atlas of Anatomy (Tank, Patrick W.; Gest, Thomas R)