Most likely you know which hand is your dominant one. This will be the hand you write with and the hand that you naturally reach out with to pick up things. You'll find that it's the hand you have the most precise control over, and without training you'll likely struggle with fine motor skills when using the non-dominant side.
But while we're aware of our dominant hands, most people don't even realise that they have a dominant eye as well, let alone know which one it is. While you might think that this doesn't matter, it can actually be a limiting factor and impact negatively on your performance in sports and in life.
Identifying Your Dominant Eye
When you look at something with both eyes open, you will only see one image. This image is an image that your brain creates from the information of both eyes and that means that your brain has to deal with the 'parallax' that comes from the two visual streams. In order to decide where objects are in space, it needs to resolve the information coming in from both eyes, and to do this it will focus predominantly on your stronger eye.
You can witness this yourself in order to try and establish your own 'oculus dominance' (also called 'eyedness'). Simply take your hands and create a circle using your thumbs and index fingers. Stretch your arms and move the circle at the end around a point of interest so that an object of some kind is located right in the centre of your cross hairs.
Now try closing each eye and watch as the object moves. With your dominant eye open you'll see that the object remains right in your sights, but when you close that eye and open your non-dominant eye, you'll see that the object 'jumps' and moves out of alignment.
So now you know which eye is feeding you the positional data and which eye is your dominant one.
So why does it matter which eye is your dominant eye? Well one reason this is important is that it will determine your ability to aim. Often when aiming we will close one eye, but if you are currently closing the wrong eye then you'll find that you can't line up your shots. Likewise if you are 'cross dominant' (meaning that your left eye is dominant and your right arm or vice versa) then you should bring your arm in more to the centre when throwing/shooting/catching as that way you will be able to better account for the fact that your aim is slightly biased to that side. You may even choose to use your other arm to help you aim on the side that you see most accurately with.
It is also possible to change the dominance of the eye by suppressing the dominant eye in order to train the non-dominant side. This can be achieved with an eye patch which forces the brain to rely on the suppressed side for its visual information. It can also be achieved through laser surgery.