Well... There is no real rule of thumb for tensioners and guide rollers.... A bad one that has seen better days can look exactly the same as a good one. You have to remove the belt anyway to find that out you see.
Try and eliminate all other possibilities first like the power steering fluid for example, the simple things first even a tiny very controlled squirt of silicon spray on the alternater pulley/ power steering pully (not the belt itself or grooves though) then you will pretty much know by then exactly what it is.
Tensioners are a bit of a quandry as you simply don't know when they are going to give up. Some give warnings, some shatter in a heart beat. The cambelt replacement 36,000 mile guideline are just that, guidelines!
Error on th side of caution mate, it will pay for itself eventually.
I certainly don't wish to worry you, thats not my intention but if it does sound as if it is getting louder, get the cambelt and tensioners changed ASAP if you are in any doubt.... £350-£450 ish is far better than £1,200 for a full rebuild
Yessssss! Nero black, Black momo leather, sunroof and air con in a 156 2.0 twin spark..... = Heaven, pure heaven
Anyone can give advice on cars like "replace this part, replace that"... Start with the simplest solution and then work your way through it logically... Its a hell of a lot easier and cheaper!