It’s actually difficult to distinguish between what really is “sufi music” and what is “Classical music” in the context of Persian music, especially if you are new to the culture and don’t speak the language.
There is a lot of overlap. Any instrumental music could potentially be Sufi, however, not all music can be Sufi music.
Music with lyrics that deal with politics, romantic love, or everyday mundane things would not be heard in Sufi contexts. It depends on the purpose of the music, which is difficult to know without knowing the meaning of lyrics. However, there are some things clues that will point you in the right direction.
If the lyrics are philosophical or from some of the poets associated with Sufism like Rumi, Hafez, Attar, etc, then they could be considered Sufi music very loosely. The reason I say “loosely” is that some of the Poets like Rumi and Hafez have been used in secular contexts as well. Rumi and Hafez’s poetry scope and influence is not confined to religious or Sufi contexts.
Homayoun Shajarian, Shahram Nazeri, and the Dastan Ensemble have done some projects that have a very strong Sufi association. Shahram Nazeri especially has done a lot of Sufi music over the last 30 years, whereas before that he was mainly singing in a classical style.
Any music using the instruments Ney, Tanbour, Setar, and Daf are considered more on the Sufi side of things. Here are some examples:
These examples are completely in the Sufi music style:
Shahram nazeri; Tanbour and Daf
Davood Azad
The example below is purely modern classical Persian music by some of the greatest masters. This is one of the most epic albums of all time in Persian music by the way. A large ensemble of various instruments are used and a lot of arrangements have been used. Sufi music is more simple in sound and arrangement.
Mohammad Reza Shajarian
My Favourite Sufi albums from 3 traditions
Persian: Dastan Ensemble and Shahram Nazeri – click image to go to playlist
Arabic: Syrie: Musique des Derviches Tourneurs de Damas, Shaykh Hamza Shakkur et l’Ensemble Al-Kindi
Turkish: Sufi Music of Turkey
This one is by far my favourite albums these days. I’ve been listening to it over and over for about 6 months.