Unfortunately, not for long. The egg she lays today might be fathered by a mating two days ago or two weeks ago. The most likely father is whichever drake mated with her 3-5 days ago (the sperm that was freshest when the egg started being formed). But there are enough exceptions that I wouldn't trust "most likely" if it is important to know the exact parentage.That technically also means if they did mate already all the eggs for some time will be purebreed if I understand it correctly.
A mating today cannot fertilize today's egg (because the egg was already partly formed before the mating happened), but today's mating may fertilize tomorrow's egg and probably will fertilize the egg for the day after tomorrow.
Sometimes you can tell by looking at the ducklings (if you have a white Pekin female, and the two possible males are a White Pekin and some other color, then any not-white ducklings would have the not-white father.)No, I think I would like to know which is which for sure.
But that requires a bit of genetics research, to know which purebreds or mixes can be recognised. I don't personally know enough about duck genetics to figure out most of the possible combinations, I just know that some combinations would look different and some would not.