Red Fox have that definate redish orange color around their shoulders, even when they're younger or their coat is motley looking. They also have the black front and back legs and the backs of their ears are black. The white tipped tail is something that you'll see in a winter coat and/or an older Red Fox.
Grey Fox generally are verying shades of grey, from tip to tail. There is some orange coloring on the under belly of a Grey Fox, but it does not extend beyond the belly area. Their tails are mostly grey with streaks of black...no white tip.
Also, a Grey's tail tends to be tall and thin as opposed to the round bushy tail of a Red.
Greys are smaller than Reds...approximately 10 lbs. smaller, but that's hard to determine when only one of them is standing there.
I hope this helps.
Good hunting, Bowhunter57