Reject something the potential composer does, see what happens.
- You want a composer who doesn’t throw a hissy fit if you show your veto power.
- You want a composer for whom going back to the drawing board is no threat.
- You want a composer who is confident about the variety of choices they can provide.
There is a scene in 7 Samurai where Takashi Shimura demonstrates how to determine real Samurai from fake. He has the young disciple, Isao Kimura, hide behind the entrance of a building, then Shimura invites potential Samurai to enter. He instructs Isao to ambush the Samurai by striking them unconscious with a heavy stick as soon as they enter. The first potential Samurai, Yoshio Inaba, approaches the door suddenly stops and refuses to enter. He knows something is wrong even though he cannot see Isao hiding. Takashi starts to laugh and reassures Yoshio it is ok to enter, just testing. He tells Isao to put down the stick.
Yoshio Inaba is basically what you want in a composer.