What to look for: "The young are strong fliers upon leaving the nest, and in many cases stay with the parents for two to three weeks." (Source: Stokes Guide to Bird Behavior Vol. I) The young flicker I sighted seemed to have been on its own already, having left the family group.
Though there are no distinct field marks that help identify adult vs. young flickers, I did notice these characteristics that helped me feel that this was indeed a young bird:
* tail seemed shorter than an adult bird's
* less "spooky" then flickers typically are in my yard
* the red crescent on nape is just barely forming
* despite the black "mustache", if this is indeed a young bird, you can't tell whether it is male or female. The mustache will remain until first fall molt.
I'm open to feedback if you feel I am mistaken!