We had a serpentine belt replaced in Vegas (we know the old one was dead because the car wouldn’t run). After the replacement it started squeeling. Did they give us a crummy belt? Or could it just be on wrong? Or could they have messed something else up while making the repair? Am I making matters worse by ignoring it because I don’t know what to do? We can’t go back and ask them because we were traveling through Vegas.
Probably just loose. Tighten it up or pay a shop a few bucks to do it. Super simple, don’t worry…unless it’s not the belt squealing…
the tensioner failing can make a howling, grinding noise. if the tensioner spring is failing it will let the alternator slip eventually killing the battery.
take it to a shop and have the tensioner checked. and all the pulleys and pumps too.
DONT go to a cheapo chain store. find a decent mechanic. (preferably with some years (more than 2!) experience)
Sigh of relief! We’ll try that. Thanks.
So how do I know if the good reply or the scary reply applies to me 
Sadly my car is at a dealership right now (not such a fan, but it died on the road today with another problem - see Nissan coil question - and I happened to be next to the dealership). I don’t like to give them more ideas of things to fix, because they’re a little too willing to comply whether it needs it or not.
Sounds like resolving this problem is going to grow your shade tree mechanic skills.
Generally, if a belt is so loose it is squealing, you can feel it with your finger. Serpentine belts should feel almost as tight as a guitar string. If you can get to the fan on the front of your alternator, and you can make the pully slip by pushing one of the little metal fan blades with your thumb, the belt is too loose.
If the belt is loose, the dealer is going to notice it while working on your coil problem.
You may find that what killed your old belt was something that is failing and pulling hard on the belt. The cheapest, and most common, failure would be a tensioner bearing, but the guy who replaced the belt should have noticed if that were the case. Other spinning parts, such as the alternator, air conditioner compressor, and power steering pump are a bit more pricy.
If it’s the correct belt, there should be no more than a one-inch deflection when you push on it. With the engine off, push on the belt half-way between the nearest two pulleys. While your’s in there, make sure that the belt is on all the pulleys. It might not be the belt; a pulley might have failing bearings. Your next step might be to start the car and listen to see if you can determine where the noise comes from exactly. No loose clothing, and don’t get too close!