A visual inspection of the belt is absolutely useless unless the belt is well past it's change interval, and then, you don't need to inspect it - you need to change it.
The rubber compound in the belt hardens with age and the many heat cycles it's forced to endure. This can cause sudden failure, but maybe more importantly slippage on the pulleys meaning the ancillaries aren't operating up to snuff. Also, most of the belt's strength comes from internal cording similar to that used in your tires and there's no way of inspecting this. Because of this, realize that there is also a time interval as well as a mileage one. Gates (world's largest mfr. of drive belts) recommends swapping the belt at the mileage interval or every 5 yrs. whichever comes first.
The belt is cheap, the procedure is simple. If it's at/beyond the change interval - change it. It's unwise not to.
