Perhaps specific context is required but what is the real meaning behind this phrase?
決まる means "to be settled" or "to be decided", and 決まっている means "has been (already) settled". But as a set phrase, (～に)決まっている also means "it goes without saying ～", "needless to say" or "certainly (although there is no direct evidence/guarantee)". See JLPT N2 Grammar: に決まっている
By extension, when used as the reply to a question, 決まっているだろう/決まっているでしょう effectively sounds like "Why ask?" or "Of course, ～" or sometimes even "That's a stupid question". You can think of this as a set phrase to memorize.
決まってんだろう is a contraction of 決まっているだろう:
- 決まっているだろう: (literally) "It has been settled, hasn't it?"
- 決まってるだろう: ("teiru → teru" contraction described here)
- 決まってんだろう: ("ru → n" contraction)