Monitoring Oracle Change Data Capture Queues
I shared a shell script that will check Oracle CDC Queues to make sure they aren't going stale. If your CDC consumer has not picked up changes in 48-72 hours then something may be wrong. The scripts can be customized to alert at any interval, hopefully this will serve as a good 'nudge' to get you going in the right direction.
Everything you need is here.
If you are an OEM user you can probably grab the SQL I shared and modify it for use with OEM User Defined Metrics or alerting.