Yes. I know how to vent sex, but I was shown how by one of the best call duck breeders in WA state. Their wee little GI tracts were only recently pulled into their stomachs, typically just a few days before hatch, so you're not supposed to apply any pressure! Nonetheless if you accidentally do it can cause...well, some very bad things, as you can imagine.
What kinds of ducks are these? I swear male call ducklings have a slightly lower peep than the females. That's the correlation I've noticed after vent sexing, anyway. I couldn't tell you how it was for other duck breeds. But when it comes to 1> week old calls, that's been my experience. Older than that and their peeps sort of even out.