Thanks Immortality. I stand corrected. The markings do indicate belt wear/stretch. More than a few folks believe that the marking are for spring tension wear. Just on that point. If tensioner uses a spring ( or coil) this must obviously wear over time so how can this tensioner wear be measured other than by obvious indirect evidence such as belt slippage.
If you want to measure how well the spring load works you would need to use a torque wrench and workout how much torque is applied just as the tensioner lifts off the stop and then compare yours to a new unit.
Personally, I don't think the spring will greatly degrade. What I have seen is the mechanism starts to wear/bind. What this does sometimes is cock the tensioner arm at an angle which can force the belt to run off.
If the tensioner isn't functioning correctly I'd try using some WD40 to lubricate the mechanism and use a spanner to rotate the unit and make sure it's moving freely.