Okay I have some experience in this as I am a victim of black magic and do ruqyah. The way you find out if someone is afflicted by black magic is usually by seeing if recitation of Quran or listining to Adhan or doing other acts of worship affects these people.
The reaction to these acts of worship depend on the jinn. A strong jinn will be more resistant. My jinn did nothing when I use to:
1)prayer 6 times a day (6th prayer being 1-2 hour in night)
2)spend 25min a day memberising Quran
3)listen to surah Baqara everyday.
But when I increased my worship to:
1)prayer 6 times a day (6th prayer being 4hr-all night)
2)spend 2hr a day memberising Quran
3)recite surah yasin everyday in salat and everynight.
Then my jinn showed a reaction. There are case where a person can be possessed for year with jinn and does not know it until he/she starts increasing in acts of worship such as nafl salat.
What I feel is the best way is that you undergo an intence ruqyah programme. This way whether it is a strong jinn or weak jinn it will show up. You can increase your worship but there are certain thing you do in ruqyah that specifically attack jinn in a powerful way, so are more effective getting a reaction.
You ruqyah programme must include
1) Drinking at least 1 litre ruqyah water, with added honey and 7 black seeds. To make ruqyah water leave ~5cm air in water bottle, and recite over top Quran verses. At the end of each verse/passage blow and spittle into the water. Verse that you must include are:
Surah Fatiha x11
2:102 (to treat black magic) x11,
7:117-119 (to treat black magic) x11,
10:81-82 (to treat black magic) x11,
20:65-69 (to treat black magic) x11,
Surah Yaseen 36 (Optional),
Surah Dukhaan 44 (optional),
Any other surahs (optional),
Another way to get Quran into water is using sheet with Quran printed on them with edible ink. simple put sheets in a big bowl of water and allow the ink to run off. You can buy the whole Quran on only 8 sheets and this would be enough for 80 litre of water.
2)cover entire body with ruqyah oil before sleeping and bathing. Ruqyah oil is made the same way as ruqyah water.
3) A bath with ruqyah water, stay in bath minimum of 30 min, continually pouring water over yourself with pot/jug. You only need to take 2 litre of ruqyah water you made and add it to a full bath. (add 7 sidr leaves and hand full of sea salt to this water)
4) read surah Baqara every day.
5) do morning and evening supplications, praising an glorification of Allah.
6) Must offer 5 daily prayer in mosque
7) read tahajjid.
8) recite at least a few hour of Quran daily, surahs such as Yasin, Saffat, surah 44, surah 55, surah jinn, surah 7, surah 2, surah 3 or any other surah but the ones mentioned are known to really attack jinn's