how to check session variable in asp.net c# using javascript or jquary?

.aspx code
			$.ajax({
                    type: "POST",
                    url: "PageURL/CheckSession",
                    data: JSON.stringify({ 'SessionName': 'UserId' }),
                    contentType: "application/json; charset=utf-8",
                    datatype: "json",                    
                    success: function (result) {
                        result = result.d;
                        alert(result);                        
                    },
                    error: function (result) {
                        alert(result.d);
                    }
                });
				
c# code				
[WebMethod]
        [ScriptMethod(ResponseFormat = ResponseFormat.Json)]
        public static string CheckSession(string SessionName)
        {
			if (HttpContext.Current.Session[SessionName] != null)
                    {
                        return Convert.ToString(HttpContext.Current.Session[SessionName]);
                    }
                    else {
                        return "fail";
					}
		}