I couldn't tell for sure until they were 6months, but I had my idea. Sam always stood up tall as a chick, and had huge "streamers", Charlie was harder to sex, but is also a boy, after a lot of umming and ahhing!
Plus...the best give-away was when Sam started making this strange noise...
I have two miniatures now, including the one above.
This is Little Charlie and Big Sam, both hatched same day, same batch of eggs
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And with my quail, Tigger for comparison (Click for Bigger)
(Click for Bigger)