Is there any telling if its a pullet?

[_meels_ the chicken mom]

Are mystic onyx hard to sex? This is my 3 week old mystic onyx that (i hope) is a pullet. Is there any sure way to tell if its a girl? Shes got pretty thick legs and a big comb. Shes a little bigger than the other chicks her age. Is it a pullet? I cant have any other roos so i hope its a hen
(These pictures are age 1-3 weeks)

 

[Poultrybonkers]

Looks like a cockerel to me