Well, I've read this thread and I would like to clarify a few things.

Belts are surprisingly durable, depending on manufacturer. The belts on my honda are perfectly fine, and original, and the car is past 160K. The thing to look for is glazing on the grooves, which indicates it has been slipping, and cracks on the outside, their should be no more than 3 cracks per inch. A nissan pathfinder came into our shop that had 200K on it and the original belt.

Belt tensioners vary in design, but it is commonly a lever attached to an accessory (commonly the alternator) or is it own idler pulley. While the manufacturer will often post a torque figure for these tensioners that use a bolt attached to lever upon which the accessory or idler pulley is attached. Some demand the use of a specialist tool to measure tension via deflection. More often than not, you can eye-ball the tension through guess and check. Simply tension until you guess it is tight enough. Turn the engine over and if it squeals, you need more tension. If it is too tight, you may hear several noises depending on the accessory involved. These vary from a whirring, grinding, or hum, but can be quite different or even bizarre.