Well it's what keeps the belt tight so it's pretty important. There should be a sticker under the hood that shows how to put tbe belt on and when you buy a new one it should have pictures for it on the back of the box too.
on the 88, sometimes it's necessary to remove the vaccum pump hose ( the metal rod that gets in the way) to replace the belt. correct way is right on the radiator above the "correct ac charge procedure" sticker.
well to know if Tensioner is working is if it keep belt tight and when you pull up on it to relase tension (Like what you do when change belt) it moves it working but if it not keeping belt tight or it doesnt move when you try to move it it bad