Until they get a male sex feather on the tail....you'll have to pick them up and listen. Any quack is a girl. At 5 weeks they are not making adult sounds all the time and it's just easier to pick them up to get a quack....at 7 weeks you should be able to do this fairly quickly and easily. Just looking at this age is just not really possible. And FYI these are really feathered out for just 5-7 weeks...so are you sure of the age? Are they white already or is it the camera/computer color off? They look like they are much older with tail feathers, no yellowish fuzz...