I'm guessing the yellow one is a pekin and the little one a mallard or rouen. Is that right? The first sign will be quacking. If they are female they will quack, usually starts around 4-6 weeks. If they are male they will make a low raspy noise. The pekin will stay all white for either gender but the male will have a drake feather. That's a little curl on his tail feathers. I think that usually happens around 4 months. The little one will have different plumage depending on gender. The biggest thing being the males green head. A quick google search will give you lots of different pictures to show the difference.
