The trick to determining the orientation of the upper arm is to locate the condyles of the humerus. The 2 points of the condyles and the "bump" at the end of the ulna form a triangle the shape of which is determined by whether the arm is straight or bent.
In a bent arm, the triangle is quite pronounced.
In a staight arm, the "elbow bump" can be at, and sometimes even above, the condyles.
It's a good idea to develop a sense of proportions. In the case of the arm, the humerus is 2 scapulas (or clavicles, or sternums, or 5-eye lines) long. The ulna is 80% of the length of the humerus.
medicine student?
I'm teaching myself structural anatomy in order to animate, draw and paint the human figure better.