Here's the basic test. Note that when the text says to hold the club in your right hand, it means your TRAILING hand:
Hold a club in just your right hand, first with an extremely strong grip (where your hand is under the bottom of the handle). Make some half speed swings and notice if the face rapidly closes, stays square or tends to stay open. Do the same exercise with a neutral grip (on the side of the handle) and a weak one (with the hand more on top). The grip that produces the most square, natural-feeling release is going to be the best one for you.
"Once you've found the right one, use that as your right hand grip," says [Top50 Teacher Mike] Adams. "It dictates the direction of the hinge. It will dictate what your right elbow does, and what your path does. You don't have to think about it. Each of us has a pattern that fits us best."
You'll want to read the entire article to understand how this can help you. There's a full explanation of Sergio's grip, but the key here is that your trailing hand grip determines the direction that you'll hinge the club in your backswing -- on an upright, neutral or flat plane. It should be obvious that if you're making a flat swing but hinging the club in an upright plane, you're going to need compensations in your downswing.
And compensations just make it harder to hit good shots.
It's a good article that covers some instruction that isn't often talked about -- and does so in a clear, understandable way. Check it out!