Fox tracks are clearly canid tracks, and differ from coyote tracks very slightly in size, shape, and gait (trail tends to be narrower). Their scat can be quite unlike coyote scat, both in size and shape. So if I find a canid trail, with just the right scat I can identify it as fox. Normally, it's very hard to be sure.
But every now and then I find a track that shows the bar on the heel pad--by every now and then, of course, I mean I've observed this 3 times, or roughly 1 time out of 100 track observations. This morning was one of those times!
I haven't seen any clear fox sign since last winter (See, especially, On the Trail of the Fox)--I've missed them.