I've done it by accident ,and you can tell right away. The tail of the arrow spins around like it's trying to stir a pot of porridge.If you look at the feather ,it will have one side that the colour looks vivid and the under side of the feather will look dull or muted.Regardless of right or left wing ,if the dull side is the same on all of the feathers on the arrow ,you're good.Even if you put your feathers on straight, the structure of the feather will cause the air to flow differently over the top and bottom of the feather. Even a straight fletched arrow will spin because of this.Mother nature is a marvelous designer! If you are still unsure ,grab an un cut /full feather and take note of how it looks.