Thats what vent sexing is. you hold the duckling upside down, bend its tail at a 90˚ angle, take two fingers and spread, do not just push down!!, but spread the vent open. sometimes gently pushing down is required but you have to be careful that you don't push to hard and hurt it.
Gonna be kinda hard to tell bill color on whites considering both are pink. but might be a difference at the young age, something BBB or one of the experienced people are going to have to answer.