Yes you can check, even the manual states that while setting the TDC through the inspection hole, you check the condition of the belt (to me, this is silly since you have a very tiny hole to look through).

When replacing the older belt on my NA, the independent specialist was an expert on 944's, he told me that the belt needed to be replaced even though he didn't take it off (yes, by looking at it).

When he took off the old belt and layed it over the new belt, the new belt was thicker (much thicker)........

Guess it comes down with experience..........but checking for missing teeth and cracks are the general way of "seeing" if the belt needs to be replaced.

I like to think I can tell, but to be honest, nobody really can. There can be hidden cracks, rotting etc. They can be a lot worse than they look, and they can be a lot better than they look. It's pretty much impossible to tell.
Best rule has already been said. I'd change the belts and rollers. For a first-timer, it won't take that long. Belts and rollers are around $120 from Zims.

and again No.

Visual inspection is fine for a KNOWN GOOD belt as a check. I have seen belts that look perfectly fine (even under duress and bending) that are bad when tensioning.. at full tension they may LOOK ok..You never know. better safe than really sorry.

Andy has a nice memorial building of bad belts (maybe 7) look good visually but are ready to give up the ghost AND valves...

Time and mileage on belts should be heavily respected.

I'm with Chris, these belts are deceptive. I like how the manual states, "Inspect and replace if necessary" without telling us what we're looking for. There's been a tonne of people here with unexpected belt-failures, regardless of mileage or age. I'd have to say, when in doubt, replace it...

I'm thinking if the belt is in several pieces, you need a new one.