The book calls for 1/4" (plus or minus 1/64", like who can measure that?) slack when pressed with 4.4lbs pressure in the center of the tension side (the side without the idler wheels). A tad loose is better than a tad tight, too tight will often cause whistle or squeaking sounds.
If you've been riding it awhile since the belts were changed and they aren't making any noise, leave them alone.
Set them and forget them until it's time for the routine mileage inspection.. I believe it's every 24,000 miles. Look for tooth wear on the drive side, cracks and threads hanging off the cut edges.. Other than that, leave them alone..