Measure how much the wire's middle deflects below the horizontal length between the two endpoints. Denote this deflection with the letter y. For a long wire, the easiest way to measure may be to send a laser beam between the two supports, just clearing the bottom of the wire. Then you can measure the distance from the laser dot up to the wire's point of insertion in the support. Some lasers come with built-in levels for such work. Or you could measure the distance from support to laser on both sides of the wire, to make sure they are equal, and therefore to ensure that your laser beam is horizontal.
