When a squealing or similar noise is heard from the engine compartment, it's nearly always blamed on the serpentine belt. Often, this is the case, but not always. It's important to know what sounds can mean and how to diagnose what the real problem is so that you don't replace a serpentine belt only to have the same issue crop up again in a few weeks.
So before you remove and replace that belt, listen to the sound and try a few simple tests to make sure that it's the only problem (a squealing belt should always be replaced as the squeal usually means damage to the belt).
Squealing sound = belt slipping. Try tightening it by adjusting the tension pulley or one of the accessories (depending on your vehicle) and see if the sound goes away. Check the belt for vertical travel to be sure it's not too loose or tight.
Squealing sound (only in early morning) = likely moisture is causing the belt to slip slightly. If the sound goes away once the vehicle is warmed up or if it only occurs for a few seconds and only in the early morning or after a car wash, it's likely just moisture and not a big deal.
Belt pops off regularly = almost always due to tension not being enough or a pulley misalignment.
Chirping sound = belt slipping on only one accessory because the pulley there is misaligned. Shut off the engine and mark all of the pulleys with white out, chalk, or fingernail polish. Start the engine. If the chirping coincides with one of the white marks hitting a certain spot every time, then that's your problem. Replace the pulley and belt.
Frayed belt edge = see "chirping" above.
Polished belt edges = slipping belt. Likely needs to be tightened, but should be replaced and all pulley grooves scoured with a light sandpaper or Brillo pad to roughen smoothed (polished) areas.
Glazed belt grooves = see "polished" above.
Excessive cracking = old belt that should be replaced or a defective tensioner. Tensioner problems will likely have one of the above issues to go with it.
Whirring sound = defective bearing in one of the pulleys. Check the tensioner first (most likely culprit) then the idler pulley (also likely). Whirring may also occasionally "grind" - this indicates imminent failure.
Belt dressing should never be used on a whining belt. It's rarely going to fix the problem for long and just covers up a symptom of a deeper issue.
Serpentine Belt Cost