A tape measure isn't necessary depending on the substrate. :-)
This was one of my favorite tricky things to do for students, because I'd see their entire face light up with the realization once they got it.
All mammals, as far as I know, scatter a bit of material at the front of their step. The foot lands in the substrate, rolls forward, and then gets picked up, and that naturally causes a little bit of material to fall away from the front of the foot and end up at the front of the track. In advanced tracking, you learn to use the material at the front of tracks to determine direction of travel. Most people duck-walk a bit, or have one foot pitched out more than the other, or might make a sudden change of direction, but the material at the front of the track is a pretty reliable indicator for direction of travel.
So one exercise we'll set up is in a sandy or soft substrate, a volunteer will hop into a boxed area, change direction of travel with each step and cover each print with a plastic cone. They jump out, and then a bunch of other plastic cones get scattered into the boxed area. Students start with the first cone, and from that track -- without being able to see any other prints -- they have to determine where the next track is (which cone contains the next print).
So you drill on this a lot until looking for that toe drag becomes reflexive, and then you take them to a set of backwards-walking tracks and see how long it takes them to realize there's no toe drag in the prints -- because the foot is rolling backward from the toes.
There are a few other giveaway clues for backwards walking too that get into the finer details of pressure points and the mechanics of movement, but toe drag is the easy one.