に決（き）まっている is a strong expression which indicates that (A) is something that the speaker either strongly believes to be true (but cannot confirm), or that they believe that (A) is only natural. These meanings primarily end up being translated as ‘surely (A)’, ‘bound to be (A)’, or ‘certainly (A)’. As this structure is a combination of に and the ている-form of the う-Verb 決（き）まる ‘to decide’, the closest literal translation is ‘for (A) to be decisively the case’.
に決（き）まっている may appear directly following verbs, い-Adjectives, な-Adjectives or nouns.

• あの選手（せんしゅ）は足（あし）が速（はや）いから勝（か）つに決（き）まっている。
  That athlete is fast at running, so they are bound to win.
• いくらクリフダイビングのプロでも、あの高（たか）さからダイブするのは怖（こわ）いに決（き）まってる。
  Diving off a cliff that high is surely scary, even for a professional cliff diver.
• この家（いえ）はあの有名（ゆうめい）な建設会社（けんせつがいしゃ）が建（た）てた家（いえ）だから、丈夫（じょうぶ）に決（き）まってる。
  This house was built by that famous construction company, so it is bound to be sturdy.
• あの人（ひと）はみんなに好（す）かれているからいい人（ひと）に決（き）まっている。
  That person is liked by everyone, so he is bound to be a good person.

に決（き）まっている primarily focuses on the speaker’s subjective opinion about (A). This means that it takes into consideration their own preformed opinions or thoughts about (A), regardless of what the actual facts may be.