In this vid Mike shows how to use a dishing wheel to make sure your bicycle wheel is properly dished.
A typical dishing tool is simply a gauge that allows you to set the distance from the rim to the hub axel, and then compare those distances on the drive-side and non-drive side of the wheel. Or in the case of a front wheel, just one side to the other.
So set the distance on the drive side and flip the wheel over. If you have a gap where the dishing tool center does not touch the hub you need to pull the rim away from that side of the wheel, by tightening the spokes on the opposite side and loosening the spokes on the current side.
Quick Tip: Always start with only a quarter turn and beginning from the valve hole on the rim as a reference point.
If the gap is between the dishing tool arms and the rim, you need to pull the rim toward that side of the wheel by tightening the spokes on that side, and loosening the spokes on the opposite side.
A properly dished wheel is centered over its hub. When centered, the rim is known to be laterally (side to side) true. If the dishing tool finds one side of the hub to be further from the rim than the other it will be necessary to tighten the spokes on that side of the wheel and hub and loosen the spokes on the other. You will need a dishing tool, spoke wrench and some kind of truing stand or device (also can use front fork) Ride on amigos.