Czy jesteś wolna ? But better option is to ask: Are u in relationship? Czy jesteś w związku ? or Are u have someone? Czy masz kogoś?

There are many ways to ask such a question. Literal translation of "are you single?" is: Jesteś singlem? And this is correct question, witch will be undersood by Poles, but is not common among them. Other good options was given by @Monia. :)