We do gib checks every 500 hours on our Komatsus. The dial indicator method raj_pm described is what we use too, but I want to add something about the measurement procedure that trips people up.
When you measure gib clearance, you need to apply a known side load to the slide. Just reading the indicator with no load tells you the static clearance, but under forming load the slide deflects and the effective clearance changes. We use a hydraulic jack between the slide and frame — apply 10% of rated tonnage sideways and read the deflection. That is your working clearance, and it is always bigger than the static number.
On the preload question — toolndye_dave is right about the 1/8 turn method for a quick check, but for a proper setup we use a torque wrench on the gib adjustment screws. Our spec is 45 Nm on the M16 lock screws. Consistent torque = consistent preload = consistent parts. Hand feel varies too much between operators, especially on a Monday morning versus a Friday afternoon.