Inspect before re-use. When you start seeing multiple signs of pending failure, it will be time to assess the whole batch.
Case neck splits, Case stretch rings (pending case separation), elongation of the case, and primers that will not show enough friction to remain seated in the primer pocket, are the main signs that there is a breakdown on the horizon.
As to having your doubts and tossing it, well, a guy could skip reloading altogether, and only use factory ammo, but where's the fun in that. Or the savings.
It gets really easy to doubt everything, if you do a bunch of reading and follow ALL of it.

There are a LOT of things that could go wrong with brass, but usually they do not all go wrong at the same time.
Use, clean (not necessary to polish, but many do), inspect, reload. Repeat.
Ask around and someone may be able to provide you with a couple examples of a separating case to see what that looks like. It is pretty easy to see once you become aware of what to look for. If you can, cut one of the separating cases lengthwise, so you can see what the inside looks like. It's educational!
Case life, as above, is going to vary a lot by your usage and loads, as well as many other criteria, so about the only sure thing is to inspect before use, and use what passes the inspection.
Cheers
Trev