Sometimes chicken breast can be thick and I never know how to tell when it's cooked! How can I be certain it is cooked all the way through?
A thermometer is the only way to be sure.
For methods with a consistent level of heat (stove, oven), you should able to learn the average cooking time, and outwards cues of color and texture that match the right internal temperature. Remember that these will vary with cooking method and temperature. Internally, the meat should look opaque and white.
These are CUES of doneness, not guarantees. Use the thermometer.
For methods with inconsistent levels of heat (bbq, campfire), wait until it looks nearly done and then check with the thermometer.
Err on the safe side. Prepared properly, chicken is a lot more forgiving of overcooking than steak.
Cook chicken with someone who's done it many times before and see how they check it. Pay attention to what it looks like near the bone (or in the center of the largest pieces if there are no bones). Do this a couple times and enjoy eating with other people :)
Or use a thermometer, but make sure it's reading the temperature correctly, and that you're putting it into the thickest meat near a bone. It'll work, but I trust someone who's done it before over a gadget I've never used.