noted in manual. I use a dowl rod with a pen mark on the rod to match a convienent reference point on the engine with the rod at the center of the belt then I push on the rod with an estimated 20 lbs and note the deflection by the reference point. Adjust till it is close to the service manual deflection for a new or used belt (they have different deflections).
To estimate 20 lbs push I get the feel by pushing the rod on a bathroom scale. Simple but effective once you get the hang of it.
Don't over tighten belts because the result will be premature failure of the bearings in the alternator, water pump and AC units.