It's easy and tricky all at the same time LOL
I've got 6 scovies that are now 12 weeks old. When they were about 10 days old I figured I had 4 girls and 2 boys...luckily I was right.
Hopefully there are more than one scovie ducklings there for you to compare. I don't think it's easy at all to tell at the age of one week personally. The boys have thicker legs, larger feet and most often a bigger chest.
I've only had Muscovies for a year so I'm sure there are other more experienced folk that may be able to give you more advice.
I apologize, I thought you posted that they are 1 week old.
Do you have other ducklings/ducks at home for the new baby to hang out with when it's older? You're stronger than me if you can simply choose one scovie...LOL they're worse than potato chips for me, I can't stop at one!