This my first batch of muscovies, so I am not an expert. With mine, the biggest difference, after size, is the legs. The boys' legs are thick and sturdy-looking. The girls' legs are more slender and delicate. If you only have one, though, it might be hard to tell at this stage.
I agree with the above statement.
Muscovy ducks are easily sexed by size as males are MUCH larger than females.
But if you only have one, it may be hard to tell until it's a little older.
You can try posting a pic though. Some of us who have raised lots of muscovies would be happy to give you an educated guess!