In that case, it will only be J inventory, they will not sell C/D/Z/P if is actually booked full. In those cases, the fare class will tell you that you won't get an upgrade.

Bottom line, I think knowledge and experience about time of week and which flight is the best indicator on whether you will clear or not. For instance, don't bother trying to hope for a upgrade on the Sunday SFO-FRA flight on a regular week, or the Thursday/Friday HKG-SFO/ORD/EWR flights.

I use both to give me a rough indicator of my odds, but nothing beats experience/knowledge.