The output of SAGE is a list of short sequence tags and the number of times it is observed.  Using sequence databases a researcher can usually determine, with some confidence, from which original mRNA (and therefore which gene) the tag was extracted.
Statistical methods can be applied to tag and count lists from different samples in order to determine which genes are more highly expressed.  For example, a normal tissue sample can be compared against a corresponding tumor to determine which genes tend to be more (or less) active.