Is it grammatically correct to use "if" and "or not" in the same sentence?
It's quite normal to add "or not" with "if", especially to establish true/false nature of a single fact( Here, the occurrence/ non-occurrence of fire).
Again, since it appears that a particular incidence of fire is being referred to, the countable form, "a fire" is appropriate: 'Let's find out if there was a fire or not.'
In talking about fire incidents, we say, for example, What needs to be done if there's a fire?