To prove the CT is earthed (grounded) at exactly one location? Lift the known earth connection and then megger the CT (or CT group) secondary winding to earth. If the reading is not very high then you another earth path. Reconnect the earth afterwards!
buzzp brings up a good caution - but I would go one further and say do not try this with current going through the ct. CT circuits should not be disturbed while in service. Especially if you are not familiar with how to use simple test tools to perform this check. Open CT circuits are very hazardous and can start fires, damage/destroy equipment, or injure/kill you.
