Necessary condition analysis

Hi, I have a research problem where I have to identify a necessary condition. I have three data series, X, Y, Z. say, Z is the dependent variable and X and Y are Independent. How do i proceed to prove that among X and Y, one is a necessary condition and the other is a sufficient condition?

please suggest reference to a statistical technique or any paper that empirically tests this.
And also reference to theoretical proofs of this nature.