What is the most reliable physical characteristic to check when attempting to sex a kestrel?
Answer
Focusing on the primary feather color of the wing.
Differentiating the sexes relies heavily on the contrasting colors of the wings: blue-gray for the male versus rusty-brown for the female, which is visible even if the wings are only slightly spread.

#Videos
How to Identify Male and Female Common Kestrels - YouTube
Related Questions
What distinguishes the kestrel as a falcon species in North America?What characteristic flight behavior strongly suggests the presence of a kestrel?When viewed from a distance, what common bird size provides a useful scale comparison for the kestrel?What color are the wings of the male American Kestrel?What is the primary color of the female kestrel's wings?What shared facial feature aids in identifying both male and female kestrels?What is the most reliable physical characteristic to check when attempting to sex a kestrel?Which description accurately captures the typical vocalization of the American Kestrel?What type of habitat do kestrels favor for hunting and perching?When hunting, what are the primary prey items mentioned for the kestrel?