>From field observations there is basically no way to tell sexes by plumage without having a bird in the hand. If you were lucky enough to have a bird in the hand, its still difficult. I'm quoting a section from Pyle's Identification of North American Birds Part II section on Great-blue Heron. This is typically a reference for banders and biologists, not a typical field guide, though some birders do own it if interested in plumage details.

Pyle: Male=female by plumage aspect. If you know the subspecies you can use measurements of the ornamental plumes for sexing . Males average longer than females for these plumes during January-May but extensive variation, age and individual overlap precludes reliable sexing.

So in the field, no ability to tell by plumage. In hand some indication but can be difficult and overlap exists.

Hope this helps.

Scott Downes
downess at charter.net
Yakima WA