One symptom is an attraction and repulsion towards Jesus, holy places, holy things, holy people. People want help but something is trying to prevent them from getting it. They will often be unable to pray through the Lord's Prayer, especially the words 'deliver us from evil'. There are other signs as well.
When the disciples asked Jesus, 'Who sinned, this man or his parents?' he didn't rebuke them for asking a stupid question but explained that this situation was different. Implicitly he accepted the idea that people could be adversely affected by their ancestor's behaviour.
Strange happenings in high places and haunted houses. John Woolmer writes on real life experiences of deliverance ministry in England.
When I was teaching mathematics at Winchester College or at theological college, the idea of deliverance, and indeed of the Devil, seemed distinctly medieval.