You need to return your promise to the calling function.
islogged:function(){
var cUid=sessionService.get('uid');
alert("in loginServce, cuid is "+cUid);
var $checkSessionServer=$http.post('data/check_session.php?cUid='+cUid);
$checkSessionServer.then(function(){
alert("session check returned!");
console.log("checkSessionServer is "+$checkSessionServer);
});
return $checkSessionServer; // <-- return your promise to the calling function
}