Muscovies have a totally different look, different head shape, different bill shape. Although with ducklings, it is harder to tell, I find. And of course, not everyone knows what to look for--but you'd expect the OWNERS or people SELLING them to you to know... right?! Perhaps most obviously, muscovies don't quack! That immediately sets them apart! They quietly chirp, warble, or hiss. The warty faces on the adults are a dead giveaway too (no other duck looks anything like that)!
Someone once offered me some ducks for free. Curious, but always wary of showing too much interest in livestock sight-unseen, I probed for more info.
"I think they're Muscovies," she told me.
"Do they quack?" I asked.
"Yes. A lot."
"Well, um, uh... Muscovies don't... uh... actually quack like other ducks..." Awkward moment!
Well, sorry you didn't get what you wanted. But who knows, maybe it'll be a blessing in disguise. You never know!