From what one breeder told me is by looking at the down of their feathers, if it is a blue or grey cast to it, then yo have a blue salmon. If the down is white, then it is a Salmon.
And from their tail feathers, when you see streaks, they are lighter, not black for Blue Salmons. The Salmons are very very dark brown or black streaks. It can occur in the primary wing feathers as well. The beards of the Blue roo would be blue and the Salmon roo beards would be black.
Hope this helps!