Please don't try to vent you duckling as there's just too much danger of damage.
Yes - it's easier if there's more than one, but certainly not fool-proof! If you post a few more pictures, particularly one from above straight down on duckling that would help, but the trouble is that there's
enough variation between individuals that if I've got a group of 10 or so ducklings there will be three we say "girls" and 3 we say "boys" and that leaves 4 in the middle that we scratch our heads over even after the 4 week point when it's supposed to be determined, and I've had Muscovy for years! So even if you post more pictures, I'd take our opinion with a grain of salt.