It's really easy.

Place a box ( or sphere or whatever) infront of your model. Do your best to make it look like the eyes are looking at the box.

Select the box then select one of the eyes then go to the constraint> AIM and click it. Do the same for the other eye. The eyes will now look at the box no matter where you move it.

You can also parent constraint the box to one of the head joints on your skeleton so that the box will move with the head but will also allow you to turn off the constraint so that you can move the box independent of the head. It just depends on what you're trying to do.