I believe the catsailor store sells two brands of "rig tension guages". They are also known as a Laos Guage (I have no idea if I'm spelling that correctly). It basically has two pins that are connected by a large mechanical disadvantage and a spring such that you put the rigging cable between the pins and rotate the device so that the long foot is parallel with the cable. the amount of twist applied to the pins shows on a scale and gives you an reading of the tension on the cable.