Untill they get some age on them you can't sex them. Im sure someone from China could, but I have never met anyone in my life that could right out of the shell. When the boys start to out grow the girls, thats when you can guess. Unless other ducks they don't quack, so you cant' shake them and "quack" them. you just let them grow.
I have had Muscovy on and off since the 80s and I still don't get them perfectly right untill they are mature. Some of them will fool ya.