, and start booming ('foomphing'), and the issue will be resolved. You can often see the sac (though it'll be obscured by her puffed-up chest feathers); and if you get to pat a female in this state, you can quite clearly feel the big weird balloon under her feathers.
Note the photo: this is a 'resident' female communicating inter-territorially with another female. Somehow, gettin' hunkered down seems to help them get a 'bigger' sound. Sometimes they get even further down, into almost a sort of crouch. The sound, a splendid bass 'Fooomph,' is unmistakeable once you've heard it -- and if you hear a string of softish vocalisations in the hour or two before dawn, that's the same thing: female.
Gettin' Your Gurk on
There is a thing, though: males seeeeem to be the only ones who 'get their gurk on': they'll gurk several times, then emit a long and increasingly loud series of gurks.
But the bottom line for U.S. folks is that some sex identification comes from watching the emus interact. Even at a half a mile, though binos, if you see in autumn a pair of emus, and one is walking behind the other, that's a breeding-pair, and the one in front is the female.
SE