Incident deletion is done in batch in the background.
To verify if a specific incident has been deleted:
WHERE IsDeleted = 0
AND IncidentID = <incidentID>;
Replace <incidentID> with the Incident ID you see on the UI without the leading 0's.
You should see 0 count from the above SQL.
However, if the intention is to see incidents getting physically deleted from the database, just remove the predicate "IsDeleted = 0":
WHERE IncidentID = <incidentID>;
The outcome should be 0 rows.
It will print to the tomcat log “IncidentDeletion task finished in xxxx seconds.”. Logging would need to be set to FINE to get specifics about how many incidents were removed.