Araucanas can hatch with tails even if both parents are rumpless. With Araucanas, gender has nothing to do with whether they have tail feathers or not.
Instead, when trying to tell male from female, I would look at the following four items:
Aggression: When you try to pick them up, do they peck at your hand, or do they squat down? The submissive ones are usually females, the aggressive ones are usually males.
Size: Cockerels are often (though not always) a little bigger and have thicker legs and larger feet. By six weeks they may look gangly, whereas the females usually stay well-proportioned.
Combs: Cockerels combs are broader and start to redden earlier than pullets.
Feathering: By 6 weeks, if you lift up the feathers on the back, the cockerels will be growing in pointy saddle feathers (saddle feathers are not tail feathers -- they grow from the back), whereas the pullets will not.
P.S. If you got your birds from a feed store or hatchery, they're probably "Easter Egg Chickens" (colored egg layers with mixed backgrounds). If you got your birds from a breeder, it's more likely that you have Araucanas.
