This is to be used only as a quick guide, and is not meant to replace DNA or surgical sexing. This does not apply to the Timneh Grey.
Most aviculturists are aware of the males being darker in color. This can only be based on the fact both birds originated in the same region of Africa, or they are chicks are from the same clutch. Also hens usually have a gradual dark to light transition of grey, from neck to belly, while males have a more uniform grey in the same area.
Importers used the under-tail coverts as the guide for sexing. This should not be termed "vent area" or "ventral feathers". The under tail coverts are directly under the tail feathers and consist of about eight feathers. Feathers of the hens will be edged in grey, while males will be solid red. Males will occasionally have a "hairline" of white on the edge.
Observe your birds from about five to ten feet away, while on a perch or hanging upside down from the cage top, and flapping their wings. With this action you are able to distinguish three bands of grey on the underside of the wing. The top band is the feathers making up the ventral antebrachial coverts. The band directly below is the feathers of the minor ventral wing coverts. The last band consists of the primary remiges. In a hen these bands respectively "appear" grey, white and dark grey. The male "appears" grey, grey and dark grey. If you hold a bird, rather than viewing from five to ten feet away, and study this, your eyes "see" the actual different bands and you can not easily distinguish male from female.
Generally, hen greys have very light grey primary-coverts, which are higly contrasted, where they overlap, the almost black primary wing feathers. Males, on the other hand, have a gradual shade difference, so it is almost indistinguishable where the primary-coverts overlap the primary wing feathers.