Perhaps the best person to tell when a spirit has left, is the individual to whom is receiving ministry. A manifestation may take place when the demon is coming out, but it is certainly not a rule of thumb in determining if the spirit has departed. When walking a person through deliverance, I will often ask them how they are feeling, if they are experiencing pressure in a part of their body (chest, stomach, mind, etc.), confusing, unusual feelings (mental or physical) or anything else. Have they felt a release from that spirit? If they feel some release, but it’s not completely, then there are likely other similar spirits that must be driven out as well. I like to work closely with the person as they are often your best guide as to how things are going.
Once a demon leaves, there is usually a feeling of release or relief from their bondage. This is not always the case, perhaps in a case of epilepsy, where they may not be able to tell until days later because their seizures have stopped. The Holy Spirit can also give the minister a feeling of confirmation that the demon has left.