We　propose　a　scheme　for　measuring　via　resonant　interactions　an　entangled　state　of　an　atom　with　a　cavity　field.　In　the　scheme,　the　elements　of　the　Wigner　characteristic　function　matrix　in　the　atomic　basis　is　directly　related　to　the　population　of　this　atom　and　an　additional　atom　after　interacting　with　the　cavity　field　displaced　by　a　classical　source.