If in doubt, change it. Even if it "looks" ok they can still shed a few teeth and go munch, I had one fail on the wife's car a couple of years back and the remnants of the belt looked in good nick! If you don't know when it was done, then do it, then you will have some piece of mind but always check the pulleys and for a thorough job change them out too whilst you are in there, the parts are not that dear even for top quality ones!
