Jackie, other than venting, which is really the only way to sex Pekin ducklings correctly, everything else is hit and miss until they are older and then develop the voice difference and feather differences which would indicate which were the drakes and which were the ducks.
However, having said that, there will be a time when Pekin ducklings do develop a difference too in the color of their bills.
Pekin ducks will develop a light, fleshy-peach colored bill while Pekin drakes will develop a deeper orange bill.
I hope you get what you wanted! This is the downside to buying day-old ducklings. Great for imprinting, but not so good if you end up with all drakes.