Walk down the incline. If your foot jams into the front of the boot and your toes feels pinched, look for another pair. If your toes touch the end of the boot, ask the salesman for the next half-size larger. If you've managed to locate a pair that meets all the criteria above, there's a good chance that you've found a reasonably good fit for your foot. If none of the boots available meet the criteria, visit another outdoor shop.
Boots can be expensive.
Take the time to choose wisely. Your bank account and feet depend on it. Once you have your boots home, slip on the socks you intend to wear while you're hiking. Then, place a blank sheet of paper under your foot, and carefully trace an outline of your foot with a pencil. Using scissors, cut the foot outline from the paper.