How is the focal point determined when looking into a clear sky?
Accommodation is the process where the lens in the eye changes shape to focus on an object. The eyes have a default accommodation distance, called the resting point of accommodation (RPA). The RPA is the distance at which the eyes focus when there is nothing to focus on. The RPA is not identical to the acommodation state of the eye when focusing at infinity. Instead, the RPA can be measured in total darkness. There, the eyes' focal distance will equal the RPA after accommodation is complete. If the lights were then turned on, an object placed at that distance would be in clear focus. The RPA averages 30 inches (approximately 760 mm) for younger people and gets farther away with age (source: D. Ankrum). 

The RPA can be measured by using infrared optometry, which is a non-invasive technique to measure the refractive state of the eye and the accommodation (Roorda et al., 1998).  

Reference
- Roorda et al., Vis Res (1998); 38(13): 1913–24