As aart mentioned above as well Marans are usually pretty easy. I can often start picking out the roos by week 3 actually. In my opinion Speckled Sussex are also very easy and Olive Eggers are a toss up, sometimes they take after Marans in the bloodline and are also easy but I find some of the EEs are harder to tell. Polish you would definitely need to wait a bit longer though. I have pictures somewhere of my Speckled Sussex for sexing...There are 3 Polishes, a Speckled Sussex, 3 Hoover's Sapphire Olive eggers, a Cuckoo Marans, 3 Black Copper Marans, and three mixes (I think ). I've heard Polishes are sometimes difficult to sex though.