The Arabic word al-`ayn (translated as the evil eye) refers to the case when a person harms another with his eye. It starts when the person likes a thing, then his evil feelings affect it, by means of his repeated looking at the object of his jealousy. Allah commanded His Prophet Muhammad (peace and blessings be upon him) to seek refuge with Him from the envier, as He said, “And from the evil of the envier when he envies.” [Al-Falaq 113:5]

Everyone who eyes another with an evil eye is envious, but not every envier looks with an evil eye to another. The word hasid (envier) is more general in meaning than the word `a’in (one who puts the evil eye on another), so seeking refuge with Allah from the one who envies includes seeking refuge with Him from the one who puts the evil eye on another. The evil eye is like an arrow which comes from the soul of the one who envies and the one who puts the evil eye on another towards the one who is envied and on whom the evil eye is put; sometimes it hits him and sometimes it misses. If the target is exposed and unprotected, it will affect him, but if the target is cautious and armed, the arrow will have no effect

Imam Ahmad and al-Tirmidhi narrated that Asma’ bint `Umays said: “O Messenger of Allah, the children of Ja`far have been afflicted by the evil eye, shall we recite ruqyah for them?” He said, “Yes, for if anything were to overtake the divine decree it would be the evil eye.”

Abu Dawood narrated that `Aa’ishah (may Allah be pleased with her) said: The person who had put the evil eye on another would be ordered to do wudu’, then the person who had been afflicted would wash himself (with that water).

Therefore, if a person looks at something with admiration and he does not pray for barakah (blessing) for it, by saying “Baarak Allahu fihi (may Allah bless it)” or the like, and this thing is then affected in some way, then it is possible to know that this person has inflicted hasad on it.

However, it is not possible to know that a specific person is the one who has envied you, except in the circumstance described above. This applies in cases where the effect is seen straightaway. But if a long time passes before something happens, then we cannot be certain that a specific person has envied this particular thing or person.