surefire

/SH UH1 R IH0 F AY1 ER0/
adjective
  1. 1

    Guaranteed to work

    โ€œThat hat should be a sure-fire way to spot him in a crowd.โ€

Source: https://en.wiktionary.org/wiki/sure-fire, https://en.wiktionary.org/wiki/surefire