Ducks of the same breed, whether they are all white or not, can definitely be difficult to tell apart unless they have random markings like pied. The coloured tie wraps are probably the best way once they are older, and in the meantime something like what @nuthatched suggested might work. How many ducklings do you have?
I'm sure you've done some research beforehand (after all, you're here, lol!) but just in case you didn't find that info, please be aware that Pekin will need extra EXTRA niacin for their lifetimes, as they are a super heavy breed and can develop joint problems. Also, if you haven't yet, find info on the care of crested ducks. It's caused by a hole in the skull, so you'll need to make sure this one remains protected from injury and not to breed it to avoid furthering this damaging defect.