Use those non contact testers as a quick reference only. Never fully rely on them and if you plan on doing future electrical work it is worth investing in a good quality meter such as a fluke. It may seem pricey but a good meter is not only for reliable information, it's also a safety device that could potentially save your life. It's one of those tools that i dont believe you should go the cheap route on.
those pens are good but not to be trusted,i can go out under the power line coming to my house, the high voltage side & it goes crazy, as others have said , always use a multimeter.always test it on a known live circuit first to see if is working properly.