A truing jig, aka a wheel truing stand, facilitates the precise truing or building of a wheel. It consists of fork, similar to a bicycle's front fork, and callipers which allow precise measurement of the position of the rim.
It is possible to make your own truing jib. Start by getting an old bicycle fork and finding a way of supporting it. The easiest way is to find a sturdy box, and drilling a hole big enough to accommodate the fork column. The hole should be tight enough to firmly hold the fork and the wheel which will be supported. The fork is now upside down. It is possible to fashion some kind of true indicator or caliper using a nail or pencil attached on one leg of the fork. A metal strap that can be attached with a screw as a pencil or nail holder works best. I try to avoid using tape, string or rubber band since they have a tendency to move which could affect an accurate reading of the wheel's straightness. Straight wheels tend to turn smoothly without deviation when viewed from the front. I start by bringing the nail as closely as possible to the wheel rim. Turning the wheel by hand, warped wheels will occasionally touch the rim or show a large gap. Wheel spokes work by providing enough pull or release on both sides of the rim so that the rim remains straight. By loosening one spoke, the tension is strengthened on the other side and vice-versa. Ideally the spokes on both sides should be even to maintain optimum rim straightness. A sequence of tightening and loosening both sides of the spokes where necessary is the key to rim straightness.