Here is a drill I like to do to check my left hand grip ever so often.....it works to keep it in the right position.
Take hold of the golf club with your left hand so that the top of the club rests on the pad of your hand below your pinkie finger......then wrap your fingers around the club like you normally would (key is to hold the club in your fingers). Once you have your grip, hold it up in the air perpendicular to your waist and release all your fingers except your index finger......you should be able to hold the club with it resting on the pad of your hand (not in the fingers or in the middle of your palm).
Another check I use is when you are addressing the ball, only two to three knuckles on your left hand should be visible.....in your case, more than 3 are likely visible, so just adjust accordingly. You can even do this before every swing if you want.
Hope that helps.
Sean