Excerpt from The Life of St. Antony:

But the inroad and the display of the evil spirits is fraught with confusion, with din, with sounds and cryings such as the disturbance of boorish youths or robbers would occasion. From which arise fear in the heart, tumult and confusion of thought, dejection, hatred towards them who live a life of discipline, indifference, grief, remembrance of kinsfolk and fear of death, and finally desire of evil things, disregard of virtue and unsettled habits. Whenever, therefore, ye have seen ought and are afraid, if your fear is immediately taken away and in place of it comes joy unspeakable, cheerfulness, courage, renewed strength, calmness of thought and all those I named before boldness and love toward God,--take courage and pray. For joy and a settled state of soul show the holiness of him who is present. Thus Abraham beholding the Lord rejoiced; so also John at the voice of Mary, the God-bearer, leaped for gladness. But if at the appearance of any there is confusion, knocking without, worldly display, threats of death and the other things which I have already mentioned, know ye that it is an onslaught of evil spirits.