The anatomical position is defined to be the body standing upright with face and torso and feet directed forward and arms around 45 degrees away from the torso with palms facing forward. This is a standard reference in human anatomy, especially in describing body part relations (i.e. the 5th digit of the hand is medial to the umbilicus, etc.). The ulna, in anatomical position is inside or medial to the midline of the body.
An easy way in determining where the ulna and the radius are is that the ulna is in line with the pinky finger and the radius is in line with the thumb.