In this context, "inspect" may not mean what you (would logically) think it means.
Inspecting a timing belt for its CONDITION is generally an exercise in futility. There's some disassembly to be done just to get your eyes on it, and then what do you see? Nothing helpful. You can tell if it's close to new, or if it's noticeably deteriorating (e.g., has cracks on its smooth side), but a belt with 20K miles on it will look essentially the same as a belt with 80K miles on it. If it actually looks bad, it's usually PAST its replacement interval. If you want to really see if it's close to needing replacement, you have to get a good look at the toothed side to tell if the teeth are starting to separate from the body of the belt. In order to do that, it's generally necessary to remove the belt, at which point it's rather silly to not just replace it.
My info calls for inspection at 60K miles, replacement at 105K. A belt with a 105K interval is not going to have any significant wear at 40K or 60K. There's really no point in assessing its condition at those mileages. Now, if there's a bunch of oil leakage onto the belt, that will shorten its life and usually necessitate replacement. This would be caused by failure of camshaft or front crankshaft seals, but they hardly ever fail that soon. It's the SECOND timing belt that's susceptible to this -- unless those seals are (wisely) replaced along with the first timing belt. And it's conceivable that there may be damage to belt from some mechanical issue (e.g. faulty idler pulley) but that's quite rare. In general, it's very unlikely a belt inspection will reveal a concern over its condition.
The inspection mentioned is almost certainly for the TENSION of the belt. Although it has a spring-assisted tensioner, it is adjustable. It is possible for the belt to stretch, and if it has loosened significantly a readjustment of the tensioner is called for. If it gets too loose, it could jump teeth, causing poor running, no start, or possibly valve damage. It does make sense to check the belt's tension to preclude this problem.
On modern engines, timing belt failure BEFORE the recommended replacement interval is exceedingly rare. Mechanically speaking, there's no reason to replace it early. It may make sense to do so for reasons of convenience, e.g. the car's in the shop for something at 100K miles and you'd just as soon not bring it back in a few months.
ETA: Checking the tension can usually be done just by removing the upper belt cover, which is a reasonably quick procedure.
Last edited by Gary T; 08-23-2011 at 09:40 AM.