Dead and standing, that's a tough one. Green, piece of cake. If the tree has only been dead a year or two, there might still be needles, maybe. Telltale sign in a dead WF could be the branching, or lack of in the tops(WF usually die topside and move down), and cluster branching mid section. If the bark is coming off toward the top, it is definitely whiter looking than DF. You say you know almost immediately when you start cutting it on the ground. Not the safest for the next person, but you could put a cut into the tree and look at your saw chips. Once you get past the outer layer, the heartwood of a DF is most assuredly darker. Trunk bark for the two can look similiar, especially if growing in the same stand. Hope this helps.