one way is if you see the same name towards the bottom of the standby list but also on the upgrade list, this is likely (but not for certain) a non-rev listed in f
Most non-revs won't be listed in F as riding in F comes with an extra service charge (not that we'd ever get an upgrade on a CPU-eligible flight). If you see someone on both lists they're more likely positive space than non-rev.
i think there's some language that they get positive space if they try a certain number of times and cannot get on.
Nope. No such language.
The only way that can happen is if scheduling can't cover a flight and they have to positive-space someone in from wherever they might be to cover it but then it becomes a deadhead, not a commute. Usually, if you miss your commutes you lose the trip and the pay associated with it.