I don't disagree with everything that Chromeo states, but I will disagree with the dark part. Most all of the downstream kelts I have caught have been quite bright but do have that kind of pewter gray look to them. One way I have always been able to tell with the hens - their vent has a red thing sticking out of it (I know, real scientific description) and their bottom fins are scuffed and in some cases worn to the point that their are bones showing in their fins. They are also quite skinny, however, as we are seeing (or soon to see) summer runs entering some rivers, the skinniness is hard to be a 100% determining factor. I would go with the vent and the wear on the bottom fins.