Epitope mapping is the determination of the specific site on an antigen, usually a protein molecule, which an antibody binds to. The most direct way to do this is to determine the atomic structure of the antigen-antibody complex, via x-ray crystallography. There are also indirect methods to do so, using ELISA, but in general, you must already know the 3D structure of the antigen. via x-ray crystallography or protein NMR. HTH.
P.S. -- Another way to do epitope mapping is the computational route: from the 3D structure of the protein antigen, predict ligand binding sites on it using computational algorithms like the one I devised (uploaded here on RG and currently in press), or those from other research groups. HTH.