khaki Campbell drakes are like mallards without the white ring around the neck. Some kC males get a little patch of white at the throat but that is "frowned on" by show breeders.
Another thing to watch for besides the curl "sex" tail feathers, the softer less quacky voice, and the mallard-colored green head, is the "eclipse" phase drakes go through after the breeding season. They will often molt their male-colored feathers and a new plumage that is a soft seal-brown (no mallard-green head), more like female coloring, will replace it until the next molt. Very strange, but it happens. But they will always have the curl in the tail feathers that identifies them as drakes, and the soft raspy voice.