Could you give an example of a sentence where you're not sure? Off the top of my head I can't really think of a good use of "wohin" with either, it just stands alone: "Wohin gehst du?"

Usually, to answer a question with 'wohin' you need the accusative. Wohin gehst du? Ich gehe in den Supermarkt (Akk.), ins Kino (Akk.), an den Strand (Akk.) That is the rule. This rule has one exception, which is the preposition 'zu', because it always requires dative. Wohin gehst du? Ich gehe zur Post (Dat.), zum Strand (Dat.).