In Warhammer, distances are measured in inches (") with a tape measure. You can always measure distances and range at any time and for any reasons, which is especially useful before you declare an action, such as charging or shooting.
This allows you to check whether your units are in range of their target before they launch an attack. After all, our warriors are all led by experienced campaigners and we can assume that they can accurately judge the range of their weapons, even if we, their generals, cannot (one does not, after all, keep a dog and then bark themself).
Distances between models and all other objects are always measured from closest point on one base to the closest point on the other base. Distances between units are always measured to and from the closest models in each of the units (see diagram below).
Sometimes units will be mounted on movement trays for ease of use. Nevertheless, always use the model's base, and not the movement tray, as the reference point when taking your measurements. So, for example, if any part of a model's base is within 6" of the base of an enemy model, the two models are said to be within 6" of each other. Sometimes the rules will call upon a unit to move directly towards another unit, or some other feature on the battlefield. Where this is the case, draw an imaginary line between the centre of the unit and its destination, and move the unit forward along this line a number of inches equal to the distance stated.