when they start to colour you can tell the difference also. the Reds (red meated) will go a red colour on the outside instead of staying silver until they basically go black. the Whites (white meat) however seem to go more a greyish colour before starting to go dark grey then finally black. You want to catch them when they are silver as this will give you the best meat quality.
there is a way to tell what type you have but im not going to explain this on a website because it invloves getting near the gills and this should only be done if you plan to kill the fish. However for the most part reds & whites come into the rivers at seperate times of year except in the Fraser where they overlap quite a bit. If your talking the Vedder the reds will start july 1st (river opens) but get more frequent mid july and the river will be full of them into august when they taper off near end of august, the whites start showing up in early september and continue (most years) into late october(last year the springs were done by october it seemed)
the reason for the different coloured meat i believe is due to genetics. some people like to say its because of what they eat which im sure has nothing to do with it.
Tell me if you can spot which of these is a red and which is a white.