Was it perhaps this article from Cackle Hatchery?
https://www.cacklehatchery.com/22-autosex-breeds-of-chickens-and-geese/
The article states, regarding Buckeye, New Hampshire Red and Buff Orpington chicks:
“
Males have a white or cream-colored spot on each upper wing.
Females have either a dark spot on the head or dorsal stripes along the back.”
I initially took this to be true for all pure Buckeyes etc. regardless of where they were bred, but I believe it’s either only true of Cackle’s lines, or maybe just something you can potentially breed for. I’m no expert, and would love to know if anyone else knows more. As far as I have heard, these three breeds are not normally considered to be autosexing.