Truly, you don't. My rouen hen last year started quacking about 4 weeks, and I read that is pretty early too. I didn't even believe what I was hearing at first but I was happy about it. Ended up with a pair, bought two straight run. I got lucky!
You know what... my one pekin hold over is a hen, she was born with a crest. She came in a batch of pekins that we were raising for meat and she honks too. Doesn't quack.. more like a horse mixed with a horn. And boy she started doing that early on, about 3-4 weeks, HONK HONK HONK. She's a great...
Sexing by voice will be your best bet and it will happen sooner than you realize. One day a "peep" will be broken up by a "QUAK" every now and then, and that's how you know you have a girl. The boys take a little longer, and they'll be a hoarse whisper at first.
The two ducklings were rouens, and they were my gateway ducks... purchased on stupid (or brilliant) impulse from the feed store. The hen is my favorite duck to this day. Love rouens.