Flights that are good candidates for operational upgrades will show lots of availability in the cabin you want to upgrade to and very little or no availability in the cabin to which you are assigned.

So if you're booked in economy, you'd want to see H0 Q0 V0 W0 S0 T0 K0 L0 -- which would mean that there were no restricted seats available and only refundable tickets for sale. That would tell me that the flight was overbooked, and that if they were selling more it would be at the highest fares (so they wouldn't mind bumping cheapies).

In that situation, if you see C9 D9, there would be plenty of business class availability and they would likely bump folks up from coach in order to get the most bodies on the plane.

See PremEx's Guide to Operational Upgrades (scroll down to the post by PremEx)