That was my experience recently. I'd observe our 6 white muscovies daily and would change my guesses when they were 4 weeks old. At 6-7 weeks I could see that 3 were larger than the other 3. I thought the drake bills were slightly wider, their feet bigger, their legs thicker, and somehow the hens faces appeared more lady? like. Also the males would bristle up their new head feathers often, the hens only occasionally. Up to 4 weeks they all stayed really close to each other. But in the weeks after would form groups or pairs away from the others. 1 drake is aggressive to people and the other drakes. He weighed in at 9lbs week 8. The middle guy gets bloodied by him. 1 hopped into wife's hand when 3 days old, was curious from the get go, accepts petting, and is always calm. He's the only drake we named along with the hens.
Your black and mixed colored muscovies are handsome. My whites refused to go into the duck pen, I worked so hard to predator proof, right after they gained confidence in their swimming. They sleep on an 7' x9' island. I can see 'em from my window and they virtually glow in the darkness. Our homestead is packed with the full assortment of predators including mean and sneaky racoons. How do the ducklings survive? They are great divers. One can see their white bodies 2 feet under the water in the daytime. They also do a thing where they kinda flap their still developing wings and attempt to run on the top of the water. They almost achieve that with their bellies barely touching the water. For 15-20' they are fast! When they settle into a regular swim, they still might outrun their predators.
Hope you get hens.
