Stacykins,
You can tell the day they are hatched, run your finger lightly over the rump, the rumpless ones will be smooth and the tailed will have that tail bump sticking out. Think about when you buy a whole chicken at the store to cook, the tail is more than just feathers, it is an extension of the vertebrae. There can still be a few stray tail feathers even if the rump feels smooth. Depending on how much, or how little there is left of the tail, there can be varying amounts of tail too.
You can tell the day they are hatched, run your finger lightly over the rump, the rumpless ones will be smooth and the tailed will have that tail bump sticking out. Think about when you buy a whole chicken at the store to cook, the tail is more than just feathers, it is an extension of the vertebrae. There can still be a few stray tail feathers even if the rump feels smooth. Depending on how much, or how little there is left of the tail, there can be varying amounts of tail too.