you can tell based on the conjunction of the verb. in the sentence it gave us we can tell it means "do y'all have everything" because the form of "have" that it uses is the one associated with the informal plural "you" (habt) rather than the pronoun "her"
one can also usually go by context, translating it both ways and seeing which one makes sense in English [do y'all have everything" rather than "do(es) her have everything"]
I don't like the Translation b/c it should be: "Do you got everything? " This sentence is more informal for the sake of getting around the more formal, longer and gramaticaly correct way of asking this question... Anyways this "habt" is more in the context of someone getting something...