squealing Serpentine belt

My serpentine belt on my 91 300 E 4-matic started squeeling during start up-when it is cold out-it usually stops in under a minute-I was able to con my ex into taking a look at it..He is a jaguar collecting, Audi driving, MB hater-HOWEVER-for a stock broker he is a darn good mechanic.....He feels the belt is loose, and told me if I could find out where the tightening point is, he would fix it for me (we were in my cold driveway, not his heated garage) Question-where is this part located? AND I did a search under belts-does my car have a belt tensioner-and if so, should I have it replaced at this time? car has 211,000 miles on it...Thanks for any help you can give me!