Sexing a Peking

We got a new Peking duck about 3 weeks ago. He was given up to the local feed store because his person was moving and could not care for him. When we adopted him he was skinny and his beak was pale yellow. In the time he has been with us he has fattened up almost to normal size and his beak is a pretty orange. Now my question is - How do I know if he is really a he. I used to mallard. that ome is easy but Pekings are new to me. So far we have not seen him trying to get to close to the girls except for following our female around. The closest I can tell is that he is a boy because when he first came the have chased him away from the girls. I think if he was a girl Claus would have taken an interest since he is such a Don Juan. If there is a way to easily tell I would love to know for sure if we have a Louie or a a Louise.