〜に決まっている
is bound to, obviously
Plain form + に決まっている (な-adj, Noun: no だ)
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に決まっている says something is certain, with no room for doubt: そんなの、うそに決まっている, "that's obviously a lie". Literally, "it's been decided that".
It's more emphatic and more casual than に違いない, and it often sounds opinionated or dismissive: 無理に決まってるよ, "that's obviously impossible". In speech, ている shrinks to てる.
Like に違いない, it follows a plain form, and nouns and な-adjectives attach without だ.
Use it for things you're sure of from common sense, not for careful deductions. Because it's so assertive, it can sound rude when used about someone else's plans or opinions.
Examples
そんなの、うそに決まっている。
That's obviously a lie.
今から行っても、間に合わないに決まっている。
Even if we go now, we're bound to be too late.
一人で全部やるなんて、無理に決まってるよ。
Doing it all by yourself? That's obviously impossible.
高い店のほうがおいしいに決まっている。
The expensive place is bound to taste better.
子どもは甘いものが好きに決まっている。
Of course kids love sweet things.