There are so many different ways you can check your swing plane. Trying too many of these methods can over-complicate your swing, taking away the fluidity and tempo, which will compromise your ball striking.
The key is to simplify your thought process, as TG Elite Teaching Pro Adrian Fryer explains in this golf video tip. Remember that all your doing is swinging a lump of metal around your body, as Adrian puts it. Adhere to two simple checkpoints and your swing will improve considerably.