Well the answer is already at your finger tips.
CBD-cargo moves when routes are served (or not). So, if your already flying a route, even only pax, you should see a move in cargo demand. If that route your looking at is so little demand compared to potential, then its' probably not going to return to ONT with a freighter on it.
What you should be looking for is routes which you already serve that have high cargo LF. Most likely, those routes have high demand and thus, putting a freighter can shift the HC as well, providing good, easy revenue.
If you want an idea of what cargo would look like out of ONT. Jump into GW1(you get 30min to preview before being charged), select ONT as your base and study the routes the airline based in ONT has. That should help you get a picture of how things would work flying out of ONT.
Talentz
PS: make sure you BK your airline and leave the GW before your 30mins are up! ~