You should try hiking shoes on at the end of the day, when your feet are at their largest. Feet swell from walking for long periods of time, something we need to remember when fitting a hiking shoe.
If you wear insoles, bring them with you while you shop! Insoles are great for extra support while hiking but they can change the fit of a shoe. You should always test the fit of hiking shoes with your insoles already in them.
Bring your own socks. If you have a favorite pair of hiking socks you should bring them to try on with a hiking boot. Everyday socks tend to be thinner than hiking socks, which can lead to you purchasing a shoe that could rub or not fit correctly once you put your hiking socks on.
You should have enough room in the toe box to wiggle your toes.
Your heel should not lift or move when walking.
Push your toes towards the front of the shoe and tilt your knee forward. You should be able to fit one finger between the back of your heel and the back of the shoe. This allows room for swelling while hiking.