well without visauly seeing them the only way to know is look for tracks and dens.a red foxes track will be about 2 to 2 1/2 inches long and about 1 3/4 inches wide.if your seeing them in snow you problably won't see claw marks.those measurements will be for the front paws,the rears are slightly smaller.a gray foxes track is about 1 1/2 to 1 3/4 inches long and slightly longer then wide.very simalar to a reds but smaller.again the fronts are larger then the rears.when following a set of tracks keep an eay out for any scat(droppings)for both reds and grays there will most times be two pieces,one slightly longer then the other.the longest one will be about 2 to 3 inches long and about the thickness of your index finger.there will be berries and fur in a reds and mostly just fur in a grays.den sites will look like a woodchuck hole(maybe slightly larger) for both spieces.most times there will be bones near the entrance to the hole,sometimes right on the rim.the best way to tell the difference between a woodchuck hole and a fox hole is reach down inside the hole about 6 inches and grab a hand full of dirt,smell it.a fox hole will have very musky smelling dirt,a woodchucks won't.WARNIG!!!don't be reaching in there to far or you might have a hand full of pist off fur <img src=icon_smile_big.gif border=0 align=middle>hope this helps.