The colours of the existing wiring and markings where they connect to the detector PCB (which I don't see in your picture) is usually the best clue. More often than not the 'main' connections to the sounder will be red and black and the feedback wire some other 'neutral' colour like white, green or blue.
Have a look at this article, which also includes a standalone circuit to drive such an element.
https://electronics.stackexchange.co...a-piezo-buzzer