From my own research I noticed that this is only possible for certain breeds days 0-3ish. Any older and it's too late! I only know because I wanted to use it for mine but they didn't arrive until day 3 and I didn't see this until about day 10.
One of my best and earliest flyers now has a bright red comb and wattles at 5 weeks, so probably a roo. Not sure if I'd use that as a gender marker. Other than that, I'm sorry, I wish I knew as well!