Star images are used to focus. Focusing should be done in its early stages at any convenient resolution. Focusing should be done in its final stages at full resolution using a subframe of the chip. Focusing exposure time should be at least four seconds to offset atmospheric turbulence. The star is most focused when it reads the highest pixel saturation.
Focused stars look round.
You must pass perfect focus on both sides to know what perfect focus is. You can't get a good image or guide well without near perfect focus. Well, since a CCD is simply an accumulation of very tiny photocells, you use the reading of the individual photocells to tell you when the most light is concentrated at a single point, thus telling you that you have achieved perfect focus. You should open your own particular software and look through its menus as I describe the process. It should translate fairly easily to your particular setup.