moment of truth

/M OW1 M AH0 N T AH0 V T R UW0 TH/
noun
  1. 1

    A deciding instant; the time when a test determines or makes it apparent whether something will succeed.

    โ€œThe moment of truth comes when you try to start the engine you have just rebuilt.โ€

Source: https://en.wiktionary.org/wiki/moment%20of%20truth