In the late seventies a few manufactures decided to try a front wheel drive car so they shoehorned a 4 cylinder in along with a transmission and the public loved them. They had some problems, I was service manager at a Dodge dealership when the 1978 Omni came out and they had their share of new design problems but the public loved them anyway. Then in the earlier eighties they decided to put a V6 in but there was a lack of room. So the serpentine belt came into the picture, instead of 2, 3 or 4 belts do it all with one. Only one problem how do we keep the belt tight and from flopping and so came the tensioner pulley. Great idea, many a quick lube manager put their kids through college on belt sales. Gates has a great sales tool that shows the allowable cracks per inch before the belt should be replaced. Check the belt for cracks as long as none of the pieces are missing it is fine but more important it should feel soft to the touch, not hard or brittle; there should be no side wear or fraying, and it should have good black color not a grey chalky look. If any of these are present replace the belt. The appearance will tell you when it needs replacing or if it noisy replace it. If it is noisy check for leaks a small amount of anti freeze can make for a noisy belt. In over 40 years I only saw a couple of belts actually fail because the belt was badly worn.The tensioner can have several problems, lose its spring tensioner, bearing failure, or it can have a limited range of motion. All are simple to diagnosis. As with many things, fluids, tires and spark plugs among others, touch, appearance and smell can be the best judge if a part needs replacement.