Telling the of a baby zebra finch?

I'll be getting zebra finches in about 2-3 weeks. THEY ARE 1 WEEK OLD RIGHT NOW. There's 5, And I'm getting 2-3 of them. I want ALL MALES since they're colorful. So when we get there, I'm going to have to choose which ones I want, and I'm afraid I might not be able to see a distinct difference. I know to look for zebra stripes on chin, pocha dot wings, and dark beak and blushy cheeks...How many of these features will be mature on a bird?