Sometimes it may be easier to blame jinns than start to think seriously what causes our problems like should we change our own behavior (I say this in general as I don´t know what problems are in some particular case).
Waswas has two forms, that which is from the self, and that which is from shaytan. The ones from the nafs is when you are tempted to do something haram due to your desire, and the one from shaytan is when you constantly have doubts about your purity or worship. You seek refuge from shaytan in those cases and learn to control your nafs.
In regards to sorcery or jinn possession, symptoms vary from person to person, however if you know the person has been to a dodgy sheikh who reveals things of the unseen/ only known to Allah, or who has instructed a person to wear or use a taweez/amulet, then it's very likely this person is affected by black magic and/or jinn.