Our Wyandotte developed red wattles and comb before 4 weeks of age. I was sure he was a roo. Posted a picture of him on here at 8 weeks and was told Wyandottes are notoriously difficult, but that he looked roo-ish. Now at 12 weeks I'm starting to think "he" is actually a pullet. But I can't stop calling him a he. Our RIR's were easy to tell. At just a week or so old I guessed that one RIR and one Welsummer were male just based on feathering. Now at 9 weeks it seems I was dead on. The RIR has shiny feathers, pointy ones, and green in his tail. The Welsummer isn't as obvious on his own (to my untrained eye), but next to our over Welsummer I think it's a pretty sure thing. It's definitely easier to tell when you have more than one of the particular breed to compare.