This picture shows the internal parts of a steering box. The rotary valve, the worm shaft, the rack, and the pitman shaft.
This slightly blurry photo shows the 12.7:1 worm shaft. The main identifying feature is the two grooves cut into the "lands" between the ball grooves. You can see this by removing the cover off of the box and turning the steering wheel all the way to the left to expose the worm shaft inside the rack piston.
As you see by this pic, the teeth on the rack are all the same size and depth. This gives this gear set a constant ratio. A variable box would have a deeper center slot between the teeth. Also, the pitman shaft center tooth is longer as well. As you will see in the next photo, this constant ratio box has symmetrical teeth.
This shows the constant tooth size of the gear on the pitman shaft. If this was a variable ratio box, the center tooth would be much larger than the other two teeth. Again, this is easy to identify even with the box mounted in the car. Just pull the four bolts off of the top cover and rotate it around to expose the internal secrets!