My duck is way, way, smaller than yours...either that...or you have hands the size of a cherry tomato (no offence meant).
When I went back to the farm last week they had 23 of her siblings...all aged 7-8 weeks.
The larger ducklings, with T-rex style ankles, has 0 wings feathers, let alone 0 flight feathers....their wings were bare little pink limbs....not even the 1st sign of feathers where the small tufts are prickley...some were fully feathered on their backs....yet still little pink stubby wings.
When my duckling is compared to them she definately "looks" female...she is still only 1-2 cm taller than my bantam cochin and under 30% the weight of my Campbell.
This weekend she has also started making a slightly different noise...I can only describe it as a high pitched cat pur....but since I have never heard an adult female Muscovy I don't know if this is duckling speak or adult speak.
The reason I said 5 wks?! is because in all my life I have never seen a more feathered large duckling as yours. At 5 wks all the ones I have ever seen are still yellow and fluffy and are not too much bigger than my hand.
I too have been in many a position with sexing ducks and chickens...thankfully...so far over the last 2-3 years...out of 10 birds I've hand picked at 1-2 weeks old all 10 have been female, never used vent sexing....3 ducks and 7 chickens.
For chickens I use the wobble test....all roosters I have come across have two left feet and always fall off things...putting a chick on your arm and wobbling your arm seems to have a good % outcome...if the chick is clumsy and falls, or struggles to stay balanced, then it's male...this so far has been used by myself and a few people I know to correctly sex over 15 chicks with 100% accuracy.
...maybe it's just pure fluke...but it seems to work...
Ducks...no scientific method...Hmm...do you look like a Molly or a Marvin...Hmmm Molly...I'll take you
Does anyone have off hand the weight of a 7-8 week old Muscovy male and female?
Go on...you know you want to go and weigh them...(don't tell your other half about poopy duck feet on your kitchen scales

)
cheers,
Woka