The word Before in the question "Have you eaten squid BEFORE? is used as an adverb.
Explanation:
There are many uses for the word before, it can be used as a preposition, a conjunction or an adverb, just to give some examples, it depends on the place it takes in the sentence as well as the grammatical function it communicates, in this question since it is located at the end it is used as an adverb, this is one of the grammatically correct uses.
