Generally, if you try to find the subject in German you ask Wer? or Was?. Which can be roughly translated with who or what, however due to the German grammar, “who” has several translations that cannot directly be reflected in the English language (Wer — Who was that? (nominativ) Wen — Who do you ask? (akkusativ)
In your examples it would be:
(1) Davids Freund Frank hört auch gern Jazz.
Wer hört auch gerne Jazz? Who likes to listen to jazz?
Answer: (Davids Freund) Frank
If you want to get the other parts of the sentence you ask
- Wessen Freund hört gerne Jazz? (Genitiv) Answer: Davids Freund
- Wen oder was hört Davids Freund Frank gerne? (Akkusativ) Answer: Jazz
(2) Hörst du auch gern Jazz?
Wer hört gerne Jazz? Who likes to listen to jazz (i.e., who is asked?)
Answer: Du
(3) Freitags geht er oft in einen Jazzclub
Wer geht Freitags oft in einen Jazzclub? Who goes to the jazz club on Fridays?
Answer: Er (He — which is the pronoun which can be used instead of a 3rd person Singular like Frank, mein Freund, etc.)