Originally Posted by steve L.
how do I know if it is tensioned correctly?
Well, if you're really good you just grab the belt with one hand, stick your tongue out the left side of your mouth and with the other hand scratch your privates and proclaim "feels pretty good to me"
Me personally, I prefer to measure the belt tension between the idler pulley and the p/s pulley with a belt tension gauge. Not enough tension and the belt slips, too much tension and you could cause damage to the bearings of the engine's externals.
A new belt should have between 180lbs - 200lbs of tension.
A used belt should have between 140lbs - 160lbs of tension.
Gates makes an inexpensive tool called a Krikit II
for checking Ribbed V /serpentine belts they're about $20.