XML is nowadays the dominant standard used for data exchanging and representation. XML documents can be transformed into different formats by using the transformation language XSLT. XSLT was designed to present data and transform data in XML according to the output schema of the used XSLT stylesheet. In this paper we describe how to compute automatically an output schema given an XSLT stylesheet. The main objective of our work is to develop a tool that contributes to a better understanding of the XML transformation process. The first results of our prototype show that we can determine the output schemas of most common XSLT stylesheets.