You'll need a paddock stand (or some way to get the rear wheel off the deck) as normally the chain slack is checked with the weight off the rear suspension.
When there, spin the rear wheel & check the up/down slack about mid point on the bottom chain run between the sprockets, keep turning until you find the tightest spot (if there is one) as this is where you need to take the measurement. Normally the slack is about 35-40mm but check your owners handbook. This is the furtherest extent that the chain covers when you move it, measuring from top of side plate at top of movement & bottom of plate at bottom of movement. A pair of cheap calipers are handy for checking this.
If too slack (or tight), loosen axle nut, adjuster locknuts & turn adjuster bolts out a flat of a bolt head at a time on both sides until the correct slack. Once adjusted, tighten lock nuts & axle nut and recheck slack before taking off stand.
Different bikes have slightly different arrangements but the same techniques applies whatever. The owners handbook should give this sort of info or get a workshop manual which will also describe the procedure.