You've got to try them on!
Two ways to do that:
Local brick-and-mortar shop that has a good selection of shoes. A good running store would be a place to start. bring the socks you intend to wear. Find a pair that fits well, then go up at least a half size, maybe a whole size, to account for swelling on a nice, long hike. If you go to an REI, and they have one of those cute fake rock things to test the shoes, lace your shoes up snugly, stand on the slope part, grab the rail, jump up and land on one foot. Try to drive your foot as far forward in the shoe as possible. Do it with both feet, even if the sales person looks at you crazily! This will let you know if you're going to have toe smash on the downhill slopes while hiking.
No local store. Go to an online shoe store (Zappo's comes to mind). Pick a model that looks interesting, and order three of them: the size you think you wear, down a size, and up a size. Wear them at home, INSIDE, until you get a feel for them. Mail the rejects back. The return should be free - check before you order! Try other shoes the same way.Eventually, you'll find one that works for you.
Tip: Once you find one that fits perfectly, see if you can find what the manufacturer's LAST is for that shoe. The last is the model foot they build the shoe around. New Balance makes theirs public; not all companies do. Having found an excellent fit with a New Balance shoe, and finding the last, I only buy NB trail runners with that last. Perfect fit guaranteed!
Tip two: Don't expect other people's favorite shoe to fit you. Their feet are different from yours. You have to do the work yourself for your own feet.