Yes look it up...it's the feathers over the back by the tail and around the neck. Males will have pointy feathers and females will not.
But in any case, don't give up on him/her yet. You won't know for certain until she

lays an egg. Or if 'he' crows...although some hens will crow but usually not at an early age (I have an egg-laying Dominique hen that 'crows' occasionally but sounds nothing like a rooster).
(I did read the breeder's response to you. I don't know anything about genetics but it sounds interesting. Lavender Ameraucana is at the top of my wish list. So this is very educational for me too for future purposes.)
Good luck!